test.cdat

%setA set < A B C >
%setB set < X Y >

%setC_relation set[2] < 
  X A 
  X B
  Y C
>

test.cmpl

#!/usr/bin/cmpl

%data test.cdat

parameters:

  echo setA;
  echo setB;
  
  {b in setB: 
    setC[b] := set{[bi, a] in setC_relation, b = bi : a };
  }
  
  
  echo "setC[X] ";
  echo setC["X"];
  echo "setC[Y] ";
  echo setC["Y"];