GETHASH

(GETHASH <key> <hashtable> [<default>])

The first parameter must be a hash table. The function returns the value
associated with the given <key> in the given <table>, or the <default>
value if there is no record with the given key in the table. The function
is always SETF-capable.

WARNING! Unlike Common Lisp GETHASH function, this version may return the
special value of the type SExpressionLabel which is textually represented
as #<HASH_EMPTY_SLOT> if no default specified. Therefore, the only case
when the third parameter might be safely omited is when the function is
used within the first argument to SETF (in which case the returned value
doesn't matter since it gets replaced anyway). For the courious, the value
returned for empty slots is SExpressionHashTable::EmptySlotMark constant,
but there's no right way to access the constant from the Lisp code (well,
creating an empty hash and doing gethash on it will do).