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

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

<b>Default Behavior</b>

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

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

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

Topics::	block_identifier

Examples::
For example, a model specification including
\verbatim
model
  responses_pointer = 'R1'
\endverbatim
will link to a response set with 
\verbatim
  id_responses = 'R1'
\endverbatim

Theory::
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