theorem Th17:
for x be Element of REAL,i be Nat st
  1 <=i & i <= m & x <> 0 holds reproj(i,0*m).x <> 0*m
proof
   let x be Element of REAL, i be Nat;
   assume 1 <=i & i <= m & x <> 0;
   then Replace(0*m,i,x) <> 0*m by Th11;
   hence thesis by PDIFF_1:def 5;
end;
