Compute the two loci on which all valid bi-arc transition points lie.

Input parameters:

1. Pa → Bi-arc start point.
2. Ta → Bi-arc start tangent.
3. Pb → Bi-arc end point.
4. Tb → Bi-arc end tangent.

Output parameters:

1. L1 → Linear or arc-shaped locus for the family of primary bi-arc solutions.
2. L2 → Arc-shaped locus for the family of alternate bi-arc solutions. This is only defined if the primary locus is not linear.