The format of the ONLYCHILD operator is:
ONLYCHILD(Object, MustObject)
The purpose of the ONLYCHILD operator is to construct a NotSet for CheckParents when finding the parents of Object during parsing - the particular token may be known to be a noun, but the parents of the object may include verb or adverbial forms ("use", for example).
ONLYCHILD finds all the hierarchy operators that have MustObject as a child, then creates a notset from the other siblings, so only the appropriate path is used.
The operator can be used to constrain inheritance.
See NOCHILD for an operator with a similar function.