VDPR

	A Precursor is valid if and only if it satisfies the
	following conditions:

		1. It appears in the Feature_body of a Feature_declaration
		of a feature f.

		2. If the Parent_qualification part is present, its Class_name
		is the name of a parent class P of C.

		3. Among the features of C's parents, limited to features of P
		if condition 2 applies, exactly one is an effective feature
		redefined by C into f. (The class to which this feature
		belongs is called the applicable parent of the Precursor.)

		4. The unfolded form relative to the applicable parent is, as
		an Unqualified_call, argument-valid.

In addition:

		5. It is valid as an Instruction if and only if f is a
		command, and as an Expression if and only if f is a query.