s the tail position of the last bone.
        if( incTail ){
            const l = chain.last();
            pt.transformVec3( [0,l.len,0], pnts[ chain.count ].pos );
        }
    }
    // #endregion

    // #region CONSTRAINTS
    static applyDistanceConstraintLinear( pnts, iStart, iLen, iInc=1, iLenStep=-1 ){
        const dir = [0,0,0];
        let ii, a, b;

        for( let i=0; i < iLen; i++ ){
            ii = iStart + ( i * iInc ); // Index to Moving Point
            a  = pnts[ ii ];            //