_B&gt;,
|
|    etc.,
|
| if the context of the discussion requires it.
|
| If we start with a model $A$ for the language $L$
| we can always expand it to a model for the language
| $L$’ = $L$ |_| X by giving appropriate interpretations
| for the symbols in X.  If $I$’ is any interpretation for
| the symbols of X in $A$, and X is disjoint from $L$, then
| $A$’ = &lt;A, $I$ |_| $I$’&gt; is a model for $L$’.  In this case
| we say that $A$’ is an 'expansion' of $A$ to $L$’, and $A$ is
| the 'redu