fn to_op(name: &str, span: &Span) -> Result<ResOp, Syntax> {
        ResOp::try_new(name)
            .ok_or_else(|| Syntax::error(&format!("Invalid operator `{}`", name), span))
    }
Finds the corresponding [`ResOp`] for a string.
Raises a syntax error if the operator string is
invalid.
passerine/src/compiler/parse.rs