I get a different result: (SB-GMP:MPZ-POW -15546163094340153687 11)
71343137007188601997892100366328301700276848580512223779418325912539619391087839151835806430552533906423576365902228543515424033914390715645863855325463682243506883429048721668340255680906224701933014782909141753
Which
is equal to (ldb (byte 704 0) (expt -15546163094340153687 11)), so, there's a problem with sign extension.
I get a different result: 153687 11)
(SB-GMP:MPZ-POW -15546163094340
713431370071886 019978921003663 283017002768485 805122237794183 259125396193910 878391518358064 305525339064235 763659022285435 154240339143907 156458638553254 636822435068834 290487216683402 556809062247019 330147829091417 53
Which
is equal to (ldb (byte 704 0) (expt -15546163094340 153687 11)), so, there's a problem with sign extension.