(/ pi 2) as a double-float.
(/ pi 2)
double-float
graphics-ports
The constant pi-by-2 is the result of (/ cl:pi 2). It is a cl:double-float.
pi-by-2
(/ cl:pi 2)
cl:double-float
2pi fpi
CAPI User Guide and Reference Manual (Unix version) - 3 Aug 2017