Third precise definitional feature of scope in Grice's natural deduction system

The first two features then define for eta being either a logical constant or a quantifier (first feature) or a term (individual constant or bound variable) (second feature).

The third feature concerns the definition of a segment.

A segment is

"a sequence of symbols which is either (1) a formula or (2) the result of substituting subscript-preserving occurrences of variables for one or more occurences of individual constants in a formula"

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This third definitional feature is needed because it expands on the previous, second one, which uses 'segment' in its specification.