reserve m,n for Nat;
reserve i,j for Integer;
reserve S for non empty addMagma;
reserve r,r1,r2,s,s1,s2,t,t1,t2 for Element of S;
reserve G for addGroup-like non empty addMagma;
reserve e,h for Element of G;
reserve G for addGroup;
reserve f,g,h for Element of G;

theorem TH9:
  -h = -g implies h = g
proof
  assume -h = -g;
  then
A1: h + -g = 0_G by Def5;
  g + -g = 0_G by Def5;
  hence thesis by A1,Th6;
end;
