function slope2(that, t) {
  var h = that._x1 - that._x0;
  return h ? (3 * (that._y1 - that._y0) / h - t) / 2 : t;
}
function slope2(that, t) {
  var h = that._x1 - that._x0;
  return h ? (3 * (that._y1 - that._y0) / h - t) / 2 : t;
}
function slope2 ( that , t ) { var h = that . _x1 - that . _x0 ; return h ? ( 3 * ( that . _y1 - that . _y0 ) / h - t ) / 2 : t ; }
Calculate a one-sided slope.
Calculate a one - sided slope .