Introduction
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| 1.4.  Theories and Examples of Theories
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| A (first-order) theory T of $L$ is a collection of sentences of $L$.
| T is said to be 'closed' iff it is closed under the |= relation.
| In view of Table 1.3.1, this is the same as requiring that T
| be closed under |- .  Since theories are sets of sentences
| of $L$, we may apply the expressions:
|
|       a model of a theory,
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|       consistent theory,
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|       satisfiable theory,
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| as introduced in Section 1.3.
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| A theory T is called 'c