/*
 *  call-seq:
 *     dvector.acos!   ->  dvector
 *  
 *  Replace each entry x of _dvector_ with acos(x).
 *     
 *     a = Dvector[ 0.1, -0.2, 0.3 ]
 *     a.acos!   -> Dvector[ acos(0.1), acos(-0.2), acos(0.3) ]
 *     a         -> Dvector[ acos(0.1), acos(-0.2), acos(0.3) ]
 */ 
VALUE dvector_acos_bang(VALUE ary) {