Function insert_not 

pub fn insert_not(phrase: &str) -> String

Fallback negation: split the phrase after its first auxiliary and insert “not”. If the first word isn’t a recognizable aux, prepend “not “ — ungrammatical on its own but better than silently losing the negation (callers should register an antonym for this case).

Examples:

• "was modified" → "was not modified"
• "has been renamed" → "has not been renamed"
• "will break" → "will not break"
• "broke" → "not broke"