Len, aAnchored=false, bAnchored=false ){
    if( aAnchored && bAnchored ) return true;

    // ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    const dir    = vec3.sub( [0,0,0], a, b );
    const curLen = vec3.len( dir );
    if( Math.abs( curLen - atLen ) < 0.0001 ) return true;
    
    // Normalize LenConstraint in relation to current distance of DIR
    const delta = ( curLen == 0 )? atLen : ( atLen - curLen ) / curLen;  

    // ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    let aScl = 0.5
  