nl_tokens: The shared instance copier based on provided callback .
pl_tokens: function _instanceCopy ( sourceRef , copyRef , rc , copier ) { let origIndex = rc . xStack . indexOf ( sourceRef ) ; if ( origIndex === - 1 ) { rc . push ( sourceRef , copyRef ) ; forEach ( sourceRef , function ( <mask> , key ) { copier ( copyRef , value , key ) ; } ) ; rc . pop ( ) ; return copyRef ; } else return rc . yStack [ origIndex ] ; }