code: function Sweep(c, a) {
    _ASSERT && common.assert(typeof c === 'undefined');
    _ASSERT && common.assert(typeof a === 'undefined');
    this.localCenter = Vec2.zero();
    this.c = Vec2.zero();
    this.a = 0;
    this.alpha0 = 0;
    this.c0 = Vec2.zero();
    this.a0 = 0;
  }
docs: This describes the motion of a body/shape for TOI computation. Shapes are
  defined with respect to the body origin, which may not coincide with the
  center of mass. However, to support dynamics we must interpolate the center
  of mass position.
  
  @prop {Vec2} localCenter Local center of mass position
  @prop {Vec2} c World center position
  @prop {float} a World angle
  @prop {float} alpha0 Fraction of the current time step in the range [0,1], c0
  and a0 are c and a at alpha0.