FUNCTION:
  MGH_IS_CALLBACK

Purpose

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  This function determines whether a variable represents a widget
  callback, in the sense used elsewhere in my widget code. The
  function has been created and given its name to allow clearer
  event-handling code.

Calling Sequence

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  Result = mgh_is_callback(value)

Positional Parameters

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  var (input)
    The variable to be examined.

Return Value

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  This function returns 1 if the variable is a single-element
  MGH_WIDGET_CALLBACK structure, otherwise 0.
###########################################################################
  This software is provided subject to the following conditions:
  1. NIWA makes no representations or warranties regarding the
    accuracy of the software, the use to which the software may
    be put or the results to be obtained from the use of the
    software. Accordingly NIWA accepts no liability for any loss
    or damage (whether direct of indirect) incurred by any person
    through the use of or reliance on the software.
  2. NIWA is to be acknowledged as the original author of the
    software where the software is used or presented in any form.
###########################################################################

Modification History

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  Mark Hadfield, Jun 2001:
      Written.