a closed
theory of the source language. It is defined in terms of the inverse
image expression function <span
 style="font-family: arial; font-style: italic;">expr</span>(<span
 style="font-weight: bold; font-style: italic;">f</span>)<small><sup>&#8211;1</sup></small>
: <big><span style="font-family: script mt bold;">P</span></big><span
 style="font-family: arial; font-style: italic;">expr</span>(<span
 style="font-style: italic; font-weight: bold;">&#321;</span>) &#8594; <big><span
 style="font-family: scr