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 D.14.2.3 member 
Procedure from library combinat.lib (see  combinat_lib).
 
- Usage:
 - member(e,L); e def, L list
 
  - Return:
 - 1 if e is an element of L, 0 otherwise
  - Theory:
 - we require the elements involved to have a compare operator ==.
   
Example:
|   | LIB "combinat.lib";
member(1,list(1,4,5));
==> 1
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