mtwist exceptions<'memory overflow'> can be returned if there is insufficient
memory to allocate a result.
<'missing value'> and <'invalid value'> can
be returned if any floating point argument is nil or is not a
valid floating point number, unless there is a designated default
interpretation for nil as in bern and u_cont.
<'bad mtwist specification'> is returned if an
argument to the bern function is not in the range of 0 to 1, or
if any probability passed to the w_* functions is negative.
<'draw from empty list'> is returned if
an argument to the *_enum functions is nil
or if an argument to w_enum contains nil.