Dear Lewis,

Either someone's got a fancy unreleased version of GAP, or they
cheated a bit with the examples ;)

the manual just doesn't specify what will happen if one uses 'ER' with
a negative integer. At the moment the function returns the correct
result for odd, negative integers but this is indeed an undocumented
feature and will either be documented in the next patch or the
function will be changed to produce an error message and one has to
use 'EI'.

best wishes
Frank