I made the same mistake as flyliufu.
I think that such a description should be a compile error (at least a warning).
Even though the following description is grammatically valid, it is semantically invalid.
“+ 4f * p1.x * u * u * u * t”
If the above description is a compile error, the following description also causes a compile error.
As for this problem, I think that it can be avoided by annotating the add () method to indicate that the return value is optional.