font-style: italic;">mod</span>(<span
 style="font-style: italic; font-weight: bold;">&#321;</span>) &#8594; <span
 style="font-family: arial; font-style: italic;">mod</span>(<span
 style="font-weight: bold; font-style: italic;">L</span>)
maps a model of the target language to a model of the source language.
This is axiomatized in the language namespace, where it is called the
model fiber function. The type (formal attribute) function <span
 style="font-family: arial; font-style: italic;">expr</span>(<span
