theorem :: Introduction => to premiss, No.1
  (|-_IPC p) & ({r} |-_IPC q) implies {p => r} |-_IPC q
proof
A20: {} c= {p => r};
  assume
A1: (|-_IPC p) & ({r} |-_IPC q); then
A2: {p => r} |-_IPC p by A20,Th66;
   {p => r} |-_IPC p => r by Th65; then
A4: {p => r} |-_IPC r by A2,Th27;
   {r} c= {p => r} \/ {r} by XBOOLE_1:7; then
  {p => r} \/ {r} |-_IPC q by A1,Th66;
  hence thesis by A4,Th116;
end;
