Module xpath from libxml2

# ## Module xpath from libxml2

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API for the XML Path Language implementation XML Path Language implementation XPath is a language for addressing parts of an XML document, designed to be used by both XSLT and XPointer ## Table of Contents
    #define XML_XPATH_CHECKNS
    #define XML_XPATH_NOVAR
    #define xmlXPathNodeSetGetLength
    #define xmlXPathNodeSetIsEmpty
    #define xmlXPathNodeSetItem
    Structure xmlNodeSet
    struct _xmlNodeSet
    Typedef xmlNodeSet * xmlNodeSetPtr
    Structure xmlXPathAxis
    struct _xmlXPathAxis
    Typedef xmlXPathAxis * xmlXPathAxisPtr
    Structure xmlXPathCompExpr
    struct _xmlXPathCompExpr
    The content of this structure is not made public by the API.
    Typedef xmlXPathCompExpr * xmlXPathCompExprPtr
    Structure xmlXPathContext
    struct _xmlXPathContext
    Typedef xmlXPathContext * xmlXPathContextPtr
    Enum xmlXPathError
    Typedef xmlXPathFunct * xmlXPathFuncPtr
    Structure xmlXPathFunct
    struct _xmlXPathFunct
    Structure xmlXPathObject
    struct _xmlXPathObject
    Typedef xmlXPathObject * xmlXPathObjectPtr
    Enum xmlXPathObjectType
    Structure xmlXPathParserContext
    struct _xmlXPathParserContext
    Typedef xmlXPathParserContext * xmlXPathParserContextPtr
    Structure xmlXPathType
    struct _xmlXPathType
    Typedef xmlXPathType * xmlXPathTypePtr
    Structure xmlXPathVariable
    struct _xmlXPathVariable
    Typedef xmlXPathVariable * xmlXPathVariablePtr
    Function type: xmlXPathAxisFunc
    xmlXPathObjectPtr	xmlXPathAxisFunc	(xmlXPathParserContextPtr ctxt,
    						 xmlXPathObjectPtr cur)
    double	xmlXPathCastBooleanToNumber	(int val)
    xmlChar *	xmlXPathCastBooleanToString	(int val)
    int	xmlXPathCastNodeSetToBoolean	(xmlNodeSetPtr ns)
    double	xmlXPathCastNodeSetToNumber	(xmlNodeSetPtr ns)
    xmlChar *	xmlXPathCastNodeSetToString	(xmlNodeSetPtr ns)
    double	xmlXPathCastNodeToNumber	(xmlNodePtr node)
    xmlChar *	xmlXPathCastNodeToString	(xmlNodePtr node)
    int	xmlXPathCastNumberToBoolean	(double val)
    xmlChar *	xmlXPathCastNumberToString	(double val)
    int	xmlXPathCastStringToBoolean	(const xmlChar * val)
    double	xmlXPathCastStringToNumber	(const xmlChar * val)
    int	xmlXPathCastToBoolean		(xmlXPathObjectPtr val)
    double	xmlXPathCastToNumber		(xmlXPathObjectPtr val)
    xmlChar *	xmlXPathCastToString	(xmlXPathObjectPtr val)
    int	xmlXPathCmpNodes		(xmlNodePtr node1,
    					 xmlNodePtr node2)
    xmlXPathCompExprPtr	xmlXPathCompile	(const xmlChar * str)
    xmlXPathObjectPtr	xmlXPathCompiledEval	(xmlXPathCompExprPtr comp,
    						 xmlXPathContextPtr ctx)
    int	xmlXPathCompiledEvalToBoolean	(xmlXPathCompExprPtr comp,
    					 xmlXPathContextPtr ctxt)
    int	xmlXPathContextSetCache		(xmlXPathContextPtr ctxt,
    					 int active,
    					 int value,
    					 int options)
    xmlXPathObjectPtr	xmlXPathConvertBoolean	(xmlXPathObjectPtr val)
    Function type: xmlXPathConvertFunc
    int	xmlXPathConvertFunc		(xmlXPathObjectPtr obj,
    					 int type)
    xmlXPathObjectPtr	xmlXPathConvertNumber	(xmlXPathObjectPtr val)
    xmlXPathObjectPtr	xmlXPathConvertString	(xmlXPathObjectPtr val)
    xmlXPathCompExprPtr	xmlXPathCtxtCompile	(xmlXPathContextPtr ctxt,
    						 const xmlChar * str)
    xmlXPathObjectPtr	xmlXPathEval	(const xmlChar * str,
    					 xmlXPathContextPtr ctx)
    xmlXPathObjectPtr	xmlXPathEvalExpression	(const xmlChar * str,
    						 xmlXPathContextPtr ctxt)
    Function type: xmlXPathEvalFunc
    void	xmlXPathEvalFunc		(xmlXPathParserContextPtr ctxt,
    					 int nargs)
    int	xmlXPathEvalPredicate		(xmlXPathContextPtr ctxt,
    					 xmlXPathObjectPtr res)
    void	xmlXPathFreeCompExpr		(xmlXPathCompExprPtr comp)
    void	xmlXPathFreeContext		(xmlXPathContextPtr ctxt)
    void	xmlXPathFreeNodeSet		(xmlNodeSetPtr obj)
    void	xmlXPathFreeNodeSetList		(xmlXPathObjectPtr obj)
    void	xmlXPathFreeObject		(xmlXPathObjectPtr obj)
    Function type: xmlXPathFuncLookupFunc
    xmlXPathFunction	xmlXPathFuncLookupFunc	(void * ctxt,
    						 const xmlChar * name,
    						 const xmlChar * ns_uri)
    Function type: xmlXPathFunction
    void	xmlXPathFunction		(xmlXPathParserContextPtr ctxt,
    					 int nargs)
    void	xmlXPathInit			(void)
    int	xmlXPathIsInf			(double val)
    int	xmlXPathIsNaN			(double val)
    xmlXPathContextPtr	xmlXPathNewContext	(xmlDocPtr doc)
    xmlNodeSetPtr	xmlXPathNodeSetCreate	(xmlNodePtr val)
    xmlXPathObjectPtr	xmlXPathObjectCopy	(xmlXPathObjectPtr val)
    long	xmlXPathOrderDocElems		(xmlDocPtr doc)
    Function type: xmlXPathVariableLookupFunc
    xmlXPathObjectPtr	xmlXPathVariableLookupFunc	(void * ctxt,
    							 const xmlChar * name,
    							 const xmlChar * ns_uri)
## Description ### Macro: XML_XPATH_CHECKNS
    #define XML_XPATH_CHECKNS
check namespaces at compilation ### Macro: XML_XPATH_NOVAR
    #define XML_XPATH_NOVAR
forbid variables in expression ### Macro: xmlXPathNodeSetGetLength
    #define xmlXPathNodeSetGetLength
Implement a functionality similar to the DOM NodeList.length. Returns the number of nodes in the node-set. ### Macro: xmlXPathNodeSetIsEmpty
    #define xmlXPathNodeSetIsEmpty
Checks whether @ns is empty or not. Returns %TRUE if @ns is an empty node-set. ### Macro: xmlXPathNodeSetItem
    #define xmlXPathNodeSetItem
Implements a functionality similar to the DOM NodeList.item(). Returns the xmlNodePtr at the given @index in @ns or NULL if @index is out of range (0 to length-1) ### Structure xmlNodeSet
    Structure xmlNodeSet
    struct _xmlNodeSet {
        int	nodeNr	: number of nodes in the set
        int	nodeMax	: size of the array as allocated
        xmlNodePtr *	nodeTab	: array of nodes in no particular order @
    }
### Structure xmlXPathAxis
    Structure xmlXPathAxis
    struct _xmlXPathAxis {
        const xmlChar *	name	: the axis name
        xmlXPathAxisFunc	func	: the search function
    }
### Structure xmlXPathCompExpr
    Structure xmlXPathCompExpr
    struct _xmlXPathCompExpr {
    The content of this structure is not made public by the API.
    }
### Structure xmlXPathContext
    Structure xmlXPathContext
    struct _xmlXPathContext {
        xmlDocPtr	doc	: The current document
        xmlNodePtr	node	: The current node
        int	nb_variables_unused	: unused (hash table)
        int	max_variables_unused	: unused (hash table)
        xmlHashTablePtr	varHash	: Hash table of defined variables
        int	nb_types	: number of defined types
        int	max_types	: max number of types
        xmlXPathTypePtr	types	: Array of defined types
        int	nb_funcs_unused	: unused (hash table)
        int	max_funcs_unused	: unused (hash table)
        xmlHashTablePtr	funcHash	: Hash table of defined funcs
        int	nb_axis	: number of defined axis
        int	max_axis	: max number of axis
        xmlXPathAxisPtr	axis	: Array of defined axis the namespace nod
        xmlNsPtr *	namespaces	: Array of namespaces
        int	nsNr	: number of namespace in scope
        void *	user	: function to free extra variables
        int	contextSize	: the context size
        int	proximityPosition	: the proximity position extra stuff for
        int	xptr	: is this an XPointer context?
        xmlNodePtr	here	: for here()
        xmlNodePtr	origin	: for origin() the set of namespace decla
        xmlHashTablePtr	nsHash	: The namespaces hash table
        xmlXPathVariableLookupFunc	varLookupFunc	: variable lookup func
        void *	varLookupData	: variable lookup data Possibility to lin
        void *	extra	: needed for XSLT The function name and U
        const xmlChar *	function
        const xmlChar *	functionURI	: function lookup function and data
        xmlXPathFuncLookupFunc	funcLookupFunc	: function lookup func
        void *	funcLookupData	: function lookup data temporary namespac
        xmlNsPtr *	tmpNsList	: Array of namespaces
        int	tmpNsNr	: number of namespaces in scope error rep
        void *	userData	: user specific data block
        xmlStructuredErrorFunc	error	: the callback in case of errors
        xmlError	lastError	: the last error
        xmlNodePtr	debugNode	: the source node XSLT dictionary
        xmlDictPtr	dict	: dictionary if any
        int	flags	: flags to control compilation Cache for
        void *	cache
    }
### Enum xmlXPathError
    Enum xmlXPathError {
        XPATH_EXPRESSION_OK = 0
        XPATH_NUMBER_ERROR = 1
        XPATH_UNFINISHED_LITERAL_ERROR = 2
        XPATH_START_LITERAL_ERROR = 3
        XPATH_VARIABLE_REF_ERROR = 4
        XPATH_UNDEF_VARIABLE_ERROR = 5
        XPATH_INVALID_PREDICATE_ERROR = 6
        XPATH_EXPR_ERROR = 7
        XPATH_UNCLOSED_ERROR = 8
        XPATH_UNKNOWN_FUNC_ERROR = 9
        XPATH_INVALID_OPERAND = 10
        XPATH_INVALID_TYPE = 11
        XPATH_INVALID_ARITY = 12
        XPATH_INVALID_CTXT_SIZE = 13
        XPATH_INVALID_CTXT_POSITION = 14
        XPATH_MEMORY_ERROR = 15
        XPTR_SYNTAX_ERROR = 16
        XPTR_RESOURCE_ERROR = 17
        XPTR_SUB_RESOURCE_ERROR = 18
        XPATH_UNDEF_PREFIX_ERROR = 19
        XPATH_ENCODING_ERROR = 20
        XPATH_INVALID_CHAR_ERROR = 21
        XPATH_INVALID_CTXT = 22
    }
### Structure xmlXPathFunct
    Structure xmlXPathFunct
    struct _xmlXPathFunct {
        const xmlChar *	name	: the function name
        xmlXPathEvalFunc	func	: the evaluation function
    }
### Structure xmlXPathObject
    Structure xmlXPathObject
    struct _xmlXPathObject {
        xmlXPathObjectType	type
        xmlNodeSetPtr	nodesetval
        int	boolval
        double	floatval
        xmlChar *	stringval
        void *	user
        int	index
        void *	user2
        int	index2
    }
### Enum xmlXPathObjectType
    Enum xmlXPathObjectType {
        XPATH_UNDEFINED = 0
        XPATH_NODESET = 1
        XPATH_BOOLEAN = 2
        XPATH_NUMBER = 3
        XPATH_STRING = 4
        XPATH_POINT = 5
        XPATH_RANGE = 6
        XPATH_LOCATIONSET = 7
        XPATH_USERS = 8
        XPATH_XSLT_TREE = 9 : An XSLT value tree, non modifiable
    }
### Structure xmlXPathParserContext
    Structure xmlXPathParserContext
    struct _xmlXPathParserContext {
        const xmlChar *	cur	: the current char being parsed
        const xmlChar *	base	: the full expression
        int	error	: error code
        xmlXPathContextPtr	context	: the evaluation context
        xmlXPathObjectPtr	value	: the current value
        int	valueNr	: number of values stacked
        int	valueMax	: max number of values stacked
        xmlXPathObjectPtr *	valueTab	: stack of values
        xmlXPathCompExprPtr	comp	: the precompiled expression
        int	xptr	: it this an XPointer expression
        xmlNodePtr	ancestor	: used for walking preceding axis
    }
### Structure xmlXPathType
    Structure xmlXPathType
    struct _xmlXPathType {
        const xmlChar *	name	: the type name
        xmlXPathConvertFunc	func	: the conversion function
    }
### Structure xmlXPathVariable
    Structure xmlXPathVariable
    struct _xmlXPathVariable {
        const xmlChar *	name	: the variable name
        xmlXPathObjectPtr	value	: the value
    }
### Function type: xmlXPathAxisFunc
    Function type: xmlXPathAxisFunc
    xmlXPathObjectPtr	xmlXPathAxisFunc	(xmlXPathParserContextPtr ctxt,
    						 xmlXPathObjectPtr cur)
An axis traversal function. To traverse an axis, the engine calls the first time with cur == NULL and repeat until the function returns NULL indicating the end of the axis traversal.
`ctxt`: | the XPath interpreter context
`cur`: | the previous node being explored on that axis
`Returns`: | the next node in that axis or NULL if at the end of the axis.
### Function: xmlXPathCastBooleanToNumber
    double	xmlXPathCastBooleanToNumber	(int val)
Converts a boolean to its number value
`val`: | a boolean
`Returns`: | the number value
### Function: xmlXPathCastBooleanToString
    xmlChar *	xmlXPathCastBooleanToString	(int val)
Converts a boolean to its string value.
`val`: | a boolean
`Returns`: | a newly allocated string.
### Function: xmlXPathCastNodeSetToBoolean
    int	xmlXPathCastNodeSetToBoolean	(xmlNodeSetPtr ns)
Converts a node-set to its boolean value
`ns`: | a node-set
`Returns`: | the boolean value
### Function: xmlXPathCastNodeSetToNumber
    double	xmlXPathCastNodeSetToNumber	(xmlNodeSetPtr ns)
Converts a node-set to its number value
`ns`: | a node-set
`Returns`: | the number value
### Function: xmlXPathCastNodeSetToString
    xmlChar *	xmlXPathCastNodeSetToString	(xmlNodeSetPtr ns)
Converts a node-set to its string value.
`ns`: | a node-set
`Returns`: | a newly allocated string.
### Function: xmlXPathCastNodeToNumber
    double	xmlXPathCastNodeToNumber	(xmlNodePtr node)
Converts a node to its number value
`node`: | a node
`Returns`: | the number value
### Function: xmlXPathCastNodeToString
    xmlChar *	xmlXPathCastNodeToString	(xmlNodePtr node)
Converts a node to its string value.
`node`: | a node
`Returns`: | a newly allocated string.
### Function: xmlXPathCastNumberToBoolean
    int	xmlXPathCastNumberToBoolean	(double val)
Converts a number to its boolean value
`val`: | a number
`Returns`: | the boolean value
### Function: xmlXPathCastNumberToString
    xmlChar *	xmlXPathCastNumberToString	(double val)
Converts a number to its string value.
`val`: | a number
`Returns`: | a newly allocated string.
### Function: xmlXPathCastStringToBoolean
    int	xmlXPathCastStringToBoolean	(const xmlChar * val)
Converts a string to its boolean value
`val`: | a string
`Returns`: | the boolean value
### Function: xmlXPathCastStringToNumber
    double	xmlXPathCastStringToNumber	(const xmlChar * val)
Converts a string to its number value
`val`: | a string
`Returns`: | the number value
### Function: xmlXPathCastToBoolean
    int	xmlXPathCastToBoolean		(xmlXPathObjectPtr val)
Converts an XPath object to its boolean value
`val`: | an XPath object
`Returns`: | the boolean value
### Function: xmlXPathCastToNumber
    double	xmlXPathCastToNumber		(xmlXPathObjectPtr val)
Converts an XPath object to its number value
`val`: | an XPath object
`Returns`: | the number value
### Function: xmlXPathCastToString
    xmlChar *	xmlXPathCastToString	(xmlXPathObjectPtr val)
Converts an existing object to its string() equivalent
`val`: | an XPath object
`Returns`: | the allocated string value of the object, NULL in case of error. It's up to the caller to free the string memory with xmlFree().
### Function: xmlXPathCmpNodes
    int	xmlXPathCmpNodes		(xmlNodePtr node1,
    					 xmlNodePtr node2)
Compare two nodes w.r.t document order
`node1`: | the first node
`node2`: | the second node
`Returns`: | -2 in case of error 1 if first point < second point, 0 if it's the same node, -1 otherwise
### Function: xmlXPathCompile
    xmlXPathCompExprPtr	xmlXPathCompile	(const xmlChar * str)
Compile an XPath expression
`str`: | the XPath expression
`Returns`: | the xmlXPathCompExprPtr resulting from the compilation or NULL. the caller has to free the object.
### Function: xmlXPathCompiledEval
    xmlXPathObjectPtr	xmlXPathCompiledEval	(xmlXPathCompExprPtr comp,
    						 xmlXPathContextPtr ctx)
Evaluate the Precompiled XPath expression in the given context.
`comp`: | the compiled XPath expression
`ctx`: | the XPath context
`Returns`: | the xmlXPathObjectPtr resulting from the evaluation or NULL. the caller has to free the object.
### Function: xmlXPathCompiledEvalToBoolean
    int	xmlXPathCompiledEvalToBoolean	(xmlXPathCompExprPtr comp,
    					 xmlXPathContextPtr ctxt)
Applies the XPath boolean() function on the result of the given compiled expression.
`comp`: | the compiled XPath expression
`ctxt`: | the XPath context
`Returns`: | 1 if the expression evaluated to true, 0 if to false and -1 in API and internal errors.
### Function: xmlXPathContextSetCache
    int	xmlXPathContextSetCache		(xmlXPathContextPtr ctxt,
    					 int active,
    					 int value,
    					 int options)
Creates/frees an object cache on the XPath context. If activates XPath objects (xmlXPathObject) will be cached internally to be reused. @options: 0: This will set the XPath object caching: @value: This will set the maximum number of XPath objects to be cached per slot There are 5 slots for: node-set, string, number, boolean, and misc objects. Use <0 for the default number (100). Other values for @options have currently no effect.
`ctxt`: | the XPath context
`active`: | enables/disables (creates/frees) the cache
`value`: | a value with semantics dependant on @options
`options`: | options (currently only the value 0 is used)
`Returns`: | 0 if the setting succeeded, and -1 on API or internal errors.
### Function: xmlXPathConvertBoolean
    xmlXPathObjectPtr	xmlXPathConvertBoolean	(xmlXPathObjectPtr val)
Converts an existing object to its boolean() equivalent
`val`: | an XPath object
`Returns`: | the new object, the old one is freed (or the operation is done directly on @val)
### Function type: xmlXPathConvertFunc
    Function type: xmlXPathConvertFunc
    int	xmlXPathConvertFunc		(xmlXPathObjectPtr obj,
    					 int type)
A conversion function is associated to a type and used to cast the new type to primitive values.
`obj`: | an XPath object
`type`: | the number of the target type
`Returns`: | -1 in case of error, 0 otherwise
### Function: xmlXPathConvertNumber
    xmlXPathObjectPtr	xmlXPathConvertNumber	(xmlXPathObjectPtr val)
Converts an existing object to its number() equivalent
`val`: | an XPath object
`Returns`: | the new object, the old one is freed (or the operation is done directly on @val)
### Function: xmlXPathConvertString
    xmlXPathObjectPtr	xmlXPathConvertString	(xmlXPathObjectPtr val)
Converts an existing object to its string() equivalent
`val`: | an XPath object
`Returns`: | the new object, the old one is freed (or the operation is done directly on @val)
### Function: xmlXPathCtxtCompile
    xmlXPathCompExprPtr	xmlXPathCtxtCompile	(xmlXPathContextPtr ctxt,
    						 const xmlChar * str)
Compile an XPath expression
`ctxt`: | an XPath context
`str`: | the XPath expression
`Returns`: | the xmlXPathCompExprPtr resulting from the compilation or NULL. the caller has to free the object.
### Function: xmlXPathEval
    xmlXPathObjectPtr	xmlXPathEval	(const xmlChar * str,
    					 xmlXPathContextPtr ctx)
Evaluate the XPath Location Path in the given context.
`str`: | the XPath expression
`ctx`: | the XPath context
`Returns`: | the xmlXPathObjectPtr resulting from the evaluation or NULL. the caller has to free the object.
### Function: xmlXPathEvalExpression
    xmlXPathObjectPtr	xmlXPathEvalExpression	(const xmlChar * str,
    						 xmlXPathContextPtr ctxt)
Evaluate the XPath expression in the given context.
`str`: | the XPath expression
`ctxt`: | the XPath context
`Returns`: | the xmlXPathObjectPtr resulting from the evaluation or NULL. the caller has to free the object.
### Function type: xmlXPathEvalFunc
    Function type: xmlXPathEvalFunc
    void	xmlXPathEvalFunc		(xmlXPathParserContextPtr ctxt,
    					 int nargs)
An XPath evaluation function, the parameters are on the XPath context stack.
`ctxt`: | an XPath parser context
`nargs`: | the number of arguments passed to the function
### Function: xmlXPathEvalPredicate
    int	xmlXPathEvalPredicate		(xmlXPathContextPtr ctxt,
    					 xmlXPathObjectPtr res)
Evaluate a predicate result for the current node. A PredicateExpr is evaluated by evaluating the Expr and converting the result to a boolean. If the result is a number, the result will be converted to true if the number is equal to the position of the context node in the context node list (as returned by the position function) and will be converted to false otherwise; if the result is not a number, then the result will be converted as if by a call to the boolean function.
`ctxt`: | the XPath context
`res`: | the Predicate Expression evaluation result
`Returns`: | 1 if predicate is true, 0 otherwise
### Function: xmlXPathFreeCompExpr
    void	xmlXPathFreeCompExpr		(xmlXPathCompExprPtr comp)
Free up the memory allocated by @comp
`comp`: | an XPATH comp
### Function: xmlXPathFreeContext
    void	xmlXPathFreeContext		(xmlXPathContextPtr ctxt)
Free up an xmlXPathContext
`ctxt`: | the context to free
### Function: xmlXPathFreeNodeSet
    void	xmlXPathFreeNodeSet		(xmlNodeSetPtr obj)
Free the NodeSet compound (not the actual nodes !).
`obj`: | the xmlNodeSetPtr to free
### Function: xmlXPathFreeNodeSetList
    void	xmlXPathFreeNodeSetList		(xmlXPathObjectPtr obj)
Free up the xmlXPathObjectPtr @obj but don't deallocate the objects in the list contrary to xmlXPathFreeObject().
`obj`: | an existing NodeSetList object
### Function: xmlXPathFreeObject
    void	xmlXPathFreeObject		(xmlXPathObjectPtr obj)
Free up an xmlXPathObjectPtr object.
`obj`: | the object to free
### Function type: xmlXPathFuncLookupFunc
    Function type: xmlXPathFuncLookupFunc
    xmlXPathFunction	xmlXPathFuncLookupFunc	(void * ctxt,
    						 const xmlChar * name,
    						 const xmlChar * ns_uri)
Prototype for callbacks used to plug function lookup in the XPath engine.
`ctxt`: | an XPath context
`name`: | name of the function
`ns_uri`: | the namespace name hosting this function
`Returns`: | the XPath function or NULL if not found.
### Function type: xmlXPathFunction
    Function type: xmlXPathFunction
    void	xmlXPathFunction		(xmlXPathParserContextPtr ctxt,
    					 int nargs)
An XPath function. The arguments (if any) are popped out from the context stack and the result is pushed on the stack.
`ctxt`: | the XPath interprestation context
`nargs`: | the number of arguments
### Function: xmlXPathInit
    void	xmlXPathInit			(void)
Initialize the XPath environment ### Function: xmlXPathIsInf
    int	xmlXPathIsInf			(double val)
Provides a portable isinf() function to detect whether a double is a +Infinite or -Infinite. Based on trio code http://sourceforge.net/projects/ctrio/
`val`: | a double value
`Returns`: | 1 vi the value is +Infinite, -1 if -Infinite, 0 otherwise
### Function: xmlXPathIsNaN
    int	xmlXPathIsNaN			(double val)
Provides a portable isnan() function to detect whether a double is a NotaNumber. Based on trio code http://sourceforge.net/projects/ctrio/
`val`: | a double value
`Returns`: | 1 if the value is a NaN, 0 otherwise
### Function: xmlXPathNewContext
    xmlXPathContextPtr	xmlXPathNewContext	(xmlDocPtr doc)
Create a new xmlXPathContext
`doc`: | the XML document
`Returns`: | the xmlXPathContext just allocated. The caller will need to free it.
### Function: xmlXPathNodeSetCreate
    xmlNodeSetPtr	xmlXPathNodeSetCreate	(xmlNodePtr val)
Create a new xmlNodeSetPtr of type double and of value @val
`val`: | an initial xmlNodePtr, or NULL
`Returns`: | the newly created object.
### Function: xmlXPathObjectCopy
    xmlXPathObjectPtr	xmlXPathObjectCopy	(xmlXPathObjectPtr val)
allocate a new copy of a given object
`val`: | the original object
`Returns`: | the newly created object.
### Function: xmlXPathOrderDocElems
    long	xmlXPathOrderDocElems		(xmlDocPtr doc)
Call this routine to speed up XPath computation on static documents. This stamps all the element nodes with the document order Like for line information, the order is kept in the element->content field, the value stored is actually - the node number (starting at -1) to be able to differentiate from line numbers.
`doc`: | an input document
`Returns`: | the number of elements found in the document or -1 in case of error.
### Function type: xmlXPathVariableLookupFunc
    Function type: xmlXPathVariableLookupFunc
    xmlXPathObjectPtr	xmlXPathVariableLookupFunc	(void * ctxt,
    							 const xmlChar * name,
    							 const xmlChar * ns_uri)
Prototype for callbacks used to plug variable lookup in the XPath engine.
`ctxt`: | an XPath context
`name`: | name of the variable
`ns_uri`: | the namespace name hosting this variable
`Returns`: | the XPath object value or NULL if not found.
Daniel Veillard