reserve X for non empty CUNITSTR;
reserve a, b for Complex;
reserve x, y for Point of X;
reserve X for ComplexUnitarySpace;
reserve x, y, z, u, v for Point of X;

theorem Th19:
  x.|.(-y) = - x.|.y
proof
  x.|.(-y) = (-x).|.y by Th17;
  hence thesis by Th18;
end;
