theorem
  len pf = 3 implies
  <* Col(<*pf*>,1), Col(<*pf*>,2), Col(<*pf*>,3) *> = F2M pf
  proof
    assume
A0: len pf = 3;
    then Col(<*pf*>,1) = <* pf.1 *> & Col(<*pf*>,2) = <* pf.2 *> &
    Col(<*pf*>,3) = <* pf.3 *> by Th51;
    hence thesis by A0,DEF1;
   end;
