The fixed words constituting the idiom (in red) build a catena each time. Note that "your" is not part of the idiom in the first tree (tree a) because the possessor is variable, e.g. "He is pulling my/her/his/someone's/etc. leg". An important caveat concerning idiom catenae is that they can be broken up in the syntax, e.g. "Your leg is being pulled". The claim, however, is that these lexical items are stored as catenae in the lexicon; they do not always appear as catenae in the actual syntax.