PegsSys is an experimental system in which a user formally describes how a program is put together by means of a hierarchically structured collection of pictures, called formal dependency diagrams (FDDs). Icons in an FDD denote a wide range of data and control dependencies among the relatively coarse-grained entities contained in large programs. Dependencies considered atomic with respect to one level in a hierarchy can be decomposed into a number of dependencies at a lower level. Each dependency can be a predefined primitive of the FDD language or it can be defined by a PegaSys user in terms of the primitives. A PegsSys user is given the illusion that logical formulas do not exist, even though PegaSys reasons about them internally. This involves (1) checking whether an FDD is meaningful syntactically, (2) determining whether hierarchical refinements of an FDD are methodologically sound, and (3) deciding whether an FDD hierarchy is logically consistent with the program that it is intended to describe. The techniques used to provide these capabilities are discussed along with the logical properties that enable PegaSys to maintain the user illusion.