reserve H,S for ZF-formula,
  x for Variable,
  X,Y for set,
  i for Element of NAT,
  e,u for set;
reserve M,M1,M2 for non empty set,
  f for Function,
  v1 for Function of VAR,M1,
  v2 for Function of VAR,M2,
  F,F1,F2 for Subset of WFF,
  W for Universe,
  a,b,c for Ordinal of W,
  A,B,C for Ordinal,
  L for DOMAIN-Sequence of W,
  va for Function of VAR,L.a,
  phi,xi for Ordinal-Sequence of W;

theorem
  M |= F1 & M |= F2 implies M |= F1 \/ F2
proof
  assume
A1: M |= F1 & M |= F2;
  let H;
  assume H in F1 \/ F2;
  then H in F1 or H in F2 by XBOOLE_0:def 3;
  hence thesis by A1;
end;
