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To check the value of a symbol against a static value, equality or inequality:
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::IF:: <symbol> == <value>
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::IF:: <symbol> != <value>
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To check for the existence of a symbol, regardless of value:
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::IFDEF:: <symbol>
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To check for the non-existence of a symbol
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::IFNDEF:: <symbol>
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To choose the other boolean path after the above:
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::ELSE::
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To terminate an IF, IFDEF, IFNDEF or ELSE block:
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::ENDIF::
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Each IF, IFDEF or IFNDEF block must be terminated by an ENDIF tag.
Each IF, IFDEF or IFNDEF *may* include an ELSE block, prior to the ENDIF tag.
IF, IFDEF and IFNDEF blocks *may* nest.
The template preprocessor uses a non-standard engine so as to not interfere with the C personality templates' use of the gnu cpp preprocessor directives.