theorem Th120:
  dom (a,b) followed_by c = NAT
proof
  thus dom (a,b) followed_by c = dom(NAT --> c) \/ dom (0,1) --> (a,b) by
FUNCT_4:def 1
    .= NAT \/ dom (0,1) --> (a,b)
    .= NAT \/ {0,1} by FUNCT_4:62
    .= NAT by XBOOLE_1:12;
end;
