IsValidField:

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  Determine if a field exists and is not empty.

   Since version 2.
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   Form:
   x = IsValidField( m, s )
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   ------
   Inputs
   ------

   m     (1,1) Structure
   s     (1,:) Field

   -------
   Outputs
   -------
   x     (1,1)  1 = true

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