/*
 *  call-seq:
 *     dvector.safe_inv(cutoff)   ->  a_dvector
 *  
 *  Returns a copy of _dvector_ with each entry x replaced by sign(x)/_cutoff_ if abs(x) < _cutoff_, 1/x otherwise.
 *     
 */ 
VALUE dvector_safe_inv(VALUE ary, VALUE arg) {