theorem Th141:
  y <> w & y <> z implies
   ((x,y,w,z) --> (a,b,c,d)).y=b
proof
assume that
A1: y<>w and
A2: y<>z;
  set f=(x,y) --> (a,b),g=(w,z) --> (c,d);
A3: f.y=b by Th63;
A4: dom g = {w,z} by Th62;
A5: not y in dom g by A1,A2,A4,TARSKI:def 2;
  thus thesis by A3,A5,Th11;
end;
