theorem Th21:
  not b _|_ a & c-b _|_ a implies ProJ(a,b,c) = 1_F
proof
  assume that
A1: not b _|_ a and
A2: c-b _|_ a;
  c-(1_F)*b _|_ a & c-ProJ(a,b,c)*b _|_ a by A1,A2,Th14;
  hence thesis by A1,Th12;
end;
