1. At a computer system including one or more processors and system memory, a method for formulating a collection of set membership conditions for a set, the method comprising: an act of accessing a set definition for a specified set, the set definition defining what resources are to be included in the specified set; an act of accessing a membership condition grammar, the membership condition grammar generally indicating how to translate a set definition for a set into membership conditions indicative of membership in the set; an act of translating the accessed set definition into one or more corresponding membership conditions defined in accordance with the membership condition grammar, each membership condition including one or more membership condition statements, each membership condition statement defining a condition about a resource's attributes that is to be true for the resource to be considered for membership in the set, including for each membership condition statement: an act of decomposing a portion of the accessed set definition into a referent field, an attribute field, an operator field, and a value field that collectively represent the defined membership condition statement, the attribute field naming an attribute, the operator field indicating a relational operator, the value field representing a value, the referent field indicating a referent, the referent referring either directly to the resource currently being evaluated for membership in the set or to another resource that is related in some way to the resource that is currently being evaluated for membership in the set; and an act of storing the one or more corresponding membership conditions for use in subsequently determining resource membership in the set.