/*
 *  call-seq:
 *     dvector.as_exponent_of!(number)                ->  dvector
 *     dvector.as_exponent_of!(other)                 ->  dvector
 *  
 *  When argument is a number, this operation replaces each entry x of _dvector_ by _number_ ** x.
 *  When argument is a vector, this operation replaces each entry x of _dvector_
 *  by the corresponding entry in the _other_ vector raised to the power x.
 *     
 *     a = Dvector[ 2, -5, 12 ]
 *     a.as_exponent_of!(3.8)              -> Dvector[ 3.8 ** 2, 3.8 ** (-5), 3.8 ** 12 ]
 *     a                                   -> Dvector[ 3.8 ** 2, 3.8 ** (-5), 3.8 ** 12 ]
 *     b = Dvector[ 7.1, 4.9, -10 ]
 *     a.as_exponent_of!(b)                -> Dvector[ 7.1 ** 2, 4.9 ** (-5), (-10) ** 12 ]
 *     a                                   -> Dvector[ 7.1 ** 2, 4.9 ** (-5), (-10) ** 12 ]
 */ 
VALUE dvector_as_exponent_of_bang(VALUE ary, VALUE arg) {