code: function(t, opt) {
  
              if (t <= 0) return 0;
  
              opt = opt || {};
              var precision = (opt.precision === undefined) ? this.PRECISION : opt.precision;
              // not using opt.subdivisions
              // not using localOpt
  
              var subCurve = this.divide(t)[0];
              var subCurveLength = subCurve.length({ precision: precision });
  
              return subCurveLength;
          }
docs: Returns distance along the curve up to `t` with precision better than requested `opt.precision`. (Not using `opt.subdivisions`.)