theorem Th35:
  for z1,z2 st z1,z2 are_commutative holds exp(z1+z2)=exp(z1) *exp
  (z2) &exp(z2+z1)=exp(z2) *exp(z1) &exp(z1+z2)=exp(z2+z1) &exp(z1),exp(z2)
  are_commutative
proof
  let z1,z2 such that
A1: z1,z2 are_commutative;
A2: exp(z2+z1)=Sum((z2+z1) rExpSeq) by Def10
    .=Sum(z2 rExpSeq)*Sum(z1 rExpSeq) by A1,Lm3
    .=exp(z2)*Sum(z1 rExpSeq) by Def10
    .=exp(z2) *exp(z1) by Def10;
  exp(z1+z2)=Sum((z1+z2) rExpSeq) by Def10
    .=Sum(z1 rExpSeq)*Sum(z2 rExpSeq) by A1,Lm3
    .=exp(z1)*Sum(z2 rExpSeq) by Def10
    .=exp(z1) *exp(z2) by Def10;
  hence thesis by A2;
end;
