theorem Th140:
  w <> z implies ((x,y,w,z) --> (a,b,c,d)).w=c
proof
  assume
A1: w <> z;
A2: w<>z by A1;
  set f=(x,y) --> (a,b),g=(w,z) --> (c,d);
A3: g.w=c by A2,Th63;
A4: dom g = {w,z} by Th62;
A5: w in dom g by A4,TARSKI:def 2;
  thus thesis by A3,A5,Th13;
end;
