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is_exists                                                 (boolSyntax)
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is_exists : term -> bool

SYNOPSIS
Tests a term to see if it is an existential quantification.

DESCRIBE
If {M} has the form {?v. t} then {is_exists M} returns {true}.
If the term is not an existential quantification the result is {false}.

FAILURE
Never fails.

SEEALSO
boolSyntax.mk_exists, boolSyntax.dest_exists.

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