However, they are compositionally less rigid than some other types of multiword expressions such as, e.g., idioms (as, e.g., [to] kick the bucket) or multiword lexical units (as, e.g., President of the United States or chief inspector).
they are formed by a freely chosen word (the base), which restricts the selection of its collocate (e.g., rain restricts us to use heavy in English to express intensity).