theorem Th142:
  x <> y & x <> w & x <> z implies
  ((x,y,w,z) --> (a,b,c,d)).x=a
proof
  assume that
A1: x<>y and
A2: x<>w and
A3: x<>z;
  set f=(x,y) --> (a,b),g=(w,z) --> (c,d);
A4: f.x=a by A1,Th63;
  dom g = {w,z} by Th62; then
A5: not x in dom g by A2,A3,TARSKI:def 2;
  thus thesis by A4,A5,Th11;
end;
