Converts a Lisp double float to a C
:double type converts between a Lisp double float and the C
In LispWorks 4.4 and previous on Windows and Linux platforms, all Lisp floats are doubles. In LispWorks 7.0, there are three disjoint Lisp float types, on all platforms.
LispWorks Foreign Language Interface User Guide and Reference Manual - 16 Feb 2015