code: function findEntryOctant(tx0, ty0, tz0, txm, tym, tzm) {
  
  	let entry = 0;
  
  	// Find the entry plane.
  	if(tx0 > ty0 && tx0 > tz0) {
  
  		// YZ-plane.
  		if(tym < tx0) {
  
  			entry |= 2;
  
  		}
  
  		if(tzm < tx0) {
  
  			entry |= 1;
  
  		}
  
  	} else if(ty0 > tz0) {
  
  		// XZ-plane.
  		if(txm < ty0) {
  
  			entry |= 4;
  
  		}
  
  		if(tzm < ty0) {
  
  			entry |= 1;
  
  		}
  
  	} else {
  
  		// XY-plane.
  		if(txm < tz0) {
  
  			entry |= 4;
  
  		}
  
  		if(tym < tz0) {
  
  			entry |= 2;
  
  		}
  
  	}
  
  	return entry;
  
  }
docs: Finds the entry plane of the first octant that a ray travels through.
  
  Determining the first octant requires knowing which of the t0s is the
  largest. The tms of the other axes must also be compared against that
  largest t0.
  
  @private
  @param {Number} tx0 - Ray projection parameter.
  @param {Number} ty0 - Ray projection parameter.
  @param {Number} tz0 - Ray projection parameter.
  @param {Number} txm - Ray projection parameter mean.
  @param {Number} tym - Ray projection parameter mean.
  @param {Number} tzm - Ray projection parameter mean.
  @return {Number} The index of the first octant that the ray travels through.