Collocations are two-word MWEs.
a PTSG is a 5-tuple V, Σ, R, ♦, θ where c ∈ V are non-terminals; t ∈ Σ are terminals; e ∈ R are elementary trees; 5 ♦ ∈ V is a unique start symbol; and θ c,e ∈ θ are parameters for each tree fragment.