This seems to be another 'explode' bug.
(%i1) 4.5b0 - 7.8;
Maxima encountered a Lisp error:
Error in MFORMAT [or a callee]: -7.7999999999999998 is not of type
STRING.
Automatically continuing.
To reenable the Lisp debugger set *debugger-hook* to nil.
(%i2) trace(?explode);
(%o2) [EXPLODE]
(%i3) 4.5b0 - 7.8;
1 Enter ?EXPLODE [- 7.8]
Maxima encountered a Lisp error:
Error in MFORMAT [or a callee]: -7.7999999999999998 is not of type
STRING.
Automatically continuing.
To reenable the Lisp debugger set *debugger-hook* to nil.
(%i4) build_info();
Maxima version: 5.9.1.1cvs
Maxima build date: 8:44 11/2/2004
host type: i686-pc-mingw32
lisp-implementation-type: Kyoto Common Lisp
lisp-implementation-version: GCL 2.6.5
Barton