 of theories construction. This is
illustrated in Figure 3, where the context of theories is represented
by the boldly enclosed upper rectangle, the context of closed theories
<span style="font-family: arial; font-weight: bold;">Closed Theory</span>
is a sub-rectangle, and the context of languages <span
 style="font-family: arial; font-weight: bold;">Language</span>
is represented by the boldly enclosed lower rectangle. In Figure 3,
each point of the theory context corresponds to a theory, and each
point of