Function validate_pairs

pub fn validate_pairs(
    pairs: Pairs<'_, Rule>,
) -> Result<Vec<&str>, Vec<Error<Rule>>>

It checks the parsed grammar for common mistakes:

• using Pest keywords
• duplicate rules
• undefined rules

It returns a Result with a Vec of Errors if any of the above is found. If no errors are found, it returns the vector of names of used builtin rules.