theorem Th128:
  dom((a,b,c) --> (x,y,z)) = {a,b,c}
proof
A1: dom (a,b) --> (x,y) = {a,b} & dom(c .--> z) = {c} by Th62;
 thus dom((a,b,c) --> (x,y,z))
      = dom (a,b) --> (x,y) \/ dom(c .--> z) by Def1
     .= {a,b,c} by A1, ENUMSET1:3;
end;
