Does not build in sbcl 1.4.11 - type inference issue

[quicklisp]

http://report.quicklisp.org/2018-09-06/failure-report/utm.html has the full log. I get this:

; caught WARNING:
;   Derived type of (- SB-C::X (FLOAT SB-C::Y SB-C::X)) is (VALUES (SINGLE-FLOAT -2.7670116e19 2.7670116e19) &OPTIONAL), conflicting with its asserted type DOUBLE-FLOAT.