 reserve x,y,z for object,
   i,j,k,l,n,m for Nat,
   D,E for non empty set;
 reserve M for Matrix of D;
 reserve L for Matrix of E;
 reserve k,t,i,j,m,n for Nat,
   D for non empty set;
 reserve V for free Z_Module;
 reserve a for Element of INT.Ring,
   W for Element of V;
 reserve KL1,KL2,KL3 for Linear_Combination of V,
   X for Subset of V;
 reserve V for finite-rank free Z_Module,
   W for Element of V;
 reserve KL1,KL2,KL3 for Linear_Combination of V,
   X for Subset of V;
 reserve s for FinSequence,
   V1,V2,V3 for finite-rank free Z_Module,
   f,f1,f2 for Function of V1,V2,
   g for Function of V2,V3,
   b1 for OrdBasis of V1,
   b2 for OrdBasis of V2,
   b3 for OrdBasis of V3,
   v1,v2 for Vector of V2,
   v,w for Element of V1;
 reserve p2,F for FinSequence of V1,
   p1,d for FinSequence of INT.Ring,
   KL for Linear_Combination of V1;

theorem BLTh26:
  for V, W being non empty ModuleStr over INT.Ring, v, u being Vector of V,
  w being Vector of W, f being Form of V,W st f is additiveSAF
  holds f.(v+u,w) = f.(v,w) + f.(u,w)
  proof
    let V, W be non empty ModuleStr over INT.Ring;
    let v, w be Vector of V, y be Vector of W, f be Form of V,W;
    set F = FunctionalSAF(f,y);
    assume f is additiveSAF;
    then
    A1: F is additive;
    thus f.(v+w,y) = F.(v+w) by BLTh9
    .= F.v+F.w by A1
    .= f.(v,y) + F.w by BLTh9
    .= f.(v,y) + f.(w,y) by BLTh9;
  end;
