Blurb::
Name the variables block; helpful when there are multiple

Description::
The optional \c id_variables keyword accepts a string that uniquely
identifies this variables block.  A model can then use these variables
by specifying the same string in its \c variables_pointer
specification.

<b>Default Behavior</b>

If the \c id_variables specification is omitted, a particular
variables specification will be used by a model only if that model
does not include an \c variables_pointer and the variables block was
the last (or only) one parsed.

<b>Usage Tips</b>
- It is a best practice to always use explicit variables IDs and
  pointers to avoid confusion.

- If only one variables block exists, then \c id_variables can be
  safely omitted from the variables block (and \c variables_pointer
  omitted from the model specification(s)), since there is no
  ambiguity.

Topics::	block_identifier

Examples::
For example, a model specification including
\verbatim
model
  variables_pointer = 'V1'
\endverbatim
will link to a response set with 
\verbatim
  id_variables = 'V1'
\endverbatim

Theory::
Faq::
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