code: def _verify_cartesian(s, t):
          
          if s < 0.0 or t < 0.0 or s + t > 1.0:
              raise ValueError("Point lies outside reference triangle", s, t)
docs: Verifies that a point is in the reference triangle.
  
          I.e., checks that they sum to <= one and are each non-negative.
  
          Args:
              s (float): Parameter along the reference triangle.
              t (float): Parameter along the reference triangle.
  
          Raises:
              ValueError: If the point lies outside the reference triangle.