In the Post office game you have to stamp each word for syntactic function. Group function is stamped onto heads, subclause function onto the '['-bracket left of the first word in a given subclause.
For instance:
'Anne (S) and (CO) her (D) boyfriend (S) have (Vaux) said (Vm), [ (Od) that (SUB) this (S) is (Vm) easy (Cs) .'