reserve X,Y for set, x,y,z for object, i,j,n for natural number;
reserve
  n for non empty Nat,
  S for non empty non void n PC-correct PCLangSignature,
  L for language MSAlgebra over S,
  F for PC-theory of L,
  A,B,C,D for Formula of L;

theorem Th103:
  A\impB in F & C\impD in F implies B\impC\imp(A\impD) in F
  proof assume
    A\impB in F & C\impD in F;
    then B\impC\imp(A\impC) in F & A\impC\imp(A\impD) in F by Th101,Th102;
    hence thesis by Th45;
  end;
