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 Th20:
  (-x).|.(-y) = x.|.y
proof
  (-x).|.(-y) = - x.|.(-y) by Th18
    .= - ( - x .|. y ) by Th19;
  hence thesis;
end;
