This definition discards the principle of syntactic atoms and would thus include such frequent patterns as 'it is' etc. Adding the constraint of atomicity would eliminate such patterns but would not be sufficient to distinguish between idioms and collocations. Collocations are a string of words that co-occur under restrictions not definable by syntax nor selectional restrictions alone. These restrictions can be referred to as lexical restrictions since the selection of the lexical unit is not conceptual, thus synonyras cannot replace the collocate. The meaning of a collocation is compositional whereas the meaning of an idiom is not.
Collocations are compositional with hierarchical relations among the lexical units.
The previous structural surface definition including a head, as the mail' word in the construction, and a collocate, as the supporting word is acceptable as is. 2 Detecting Collocations in a Text A multi-word idiom often violates selectional restrictions due to metaphorical use of words whereas a collocation will not. Thus an idiom may be detected in failing parses where a collocation will be parsed undetected. Collocations are 4permitted patterns' in contrast to idioms that are often 'prohibited patterns' within the selectional restriction frame. The borderline between the two is difficult to draw.