Function find_child 

pub(crate) fn find_child<'input>(
    pair: Pair<'input, Rule>,
    rule: Rule,
    context: &str,
) -> Result<Pair<'input, Rule>, Diagnostic>

Find the first inner child of pair matching rule.

context is included in the error message to identify which builder asked for it (e.g. “If statement missing condition expression.”).