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\DOC free_in
\TYPE {free_in : (term -> term -> bool)}
\SYNOPSIS
Tests if one term is free in another.
\DESCRIBE
When applied to two terms {t1} and {t2}, the function {free_in} returns
{true} if {t1} is free in {t2}, and {false} otherwise. It is not necessary
that {t1} be simply a variable.
\FAILURE
Never fails.
\EXAMPLE
In the following example {free_in} returns {false} because the {x} in {SUC x}
in the second term is bound:
{
#free_in "SUC x" "!x. SUC x = x + 1";;
false : bool
}
\noindent whereas the following call returns {true} because the first instance
of {x} in the second term is free, even though there is also a bound instance:
{
#free_in "x:bool" "x /\ (?x. x=T)";;
true : bool
}
\SEEALSO
frees, freesl, thm_frees.
\ENDDOC
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