theorem
 f +* (x .--> z1) +* (x .--> z2) = f +* (x .--> z2)
proof
A1: dom (x .--> z1) = {x}
    .= dom (x .--> z2);
  thus f +* (x .--> z1) +* (x .--> z2)
   = f +* ((x .--> z1) +* (x .--> z2)) by Th14
    .= f +* (x .--> z2) by A1,Th19;
end;
