theorem Th134:
  a <> b & a <> c implies ((a,b,c) --> (x,y,z)).a = x
 proof assume that
A1: a <> b and
A2: a <> c;
   not a in dom(c.-->z) by A2,TARSKI:def 1;
  hence ((a,b,c) --> (x,y,z)).a = ((a,b) --> (x,y)).a by Th11
    .= x by A1,Th63;
 end;
