theorem Th137:
  dom (x,y,w,z) --> (a,b,c,d) = {x,y,w,z}
proof
  set f=(x,y) --> (a,b), g=(w,z) --> (c,d);
A1: dom f={x,y} by Th62;
  dom g={w,z} by Th62;
  then dom f \/ dom g = {x,y,w,z} by A1,ENUMSET1:5;
  hence thesis by Def1;
end;
