[Haskell-cafe] What does it mean to derive "equations of restricted from" in Haskell?

In John Hughes's "The Design of Pretty printing library" paper, he says: "The implementations which we are trying to derive consist of equations of

a restricted form. We will derive implementations by proving their constituent equations from the specification. By itself this is no guarantee that the implemented functions satisfy the specification because we might not have proved enough equation But if we also check that the derived definitions are terminating and exhaustive then this property is guaranteed"

What does "restricted form" mean? What is the meaning and significance of "definitions are terminating and exhaustive"? -- Daryoush Weblog: http://onfp.blogspot.com/