Function fold_pattern_value 

pub fn fold_pattern_value<'i, F, T>(
    folder: &mut F,
    pattern: PatternNode<'i, T>,
) -> Result<PatternNode<'i, T>, F::Error>
where
    F: ExpressionFolder<'i, T> + ?Sized,
    T: FoldableExpression<'i>,

Transforms pattern value by using folder.