Function label_add 

pub fn label_add(label: &mut [Environment], n: usize, env: Environment) -> usize

Add env to a label held in label[..n], maintaining minimality: if an existing environment already subsumes env (existing ⊆ env), env is redundant and is dropped; any existing environments that env subsumes (env ⊆ existing) are removed in favour of the more general env. Returns the new label length. Zero-heap (in-place compaction of label).