DATE     94/01/10

    Procédure ACTI3
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    Description :
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    This procedure could be used by the INCREME procedure. 
 In fact, it uses the ACT3 operator which has exactly the same
 function.

   From 4 displacement field associated with 4 incremental 
 fields of correcting forces, ACTI3 evaluates the displacement 
 incremental field that enables the solution of the problem 
 concerning the restriction of the application tangential to the
 subspace of dimension 3 defined by the fields on input.