code: def _cmp_fstruct(self, s1, s2, frac_tol, mask):
          
          if len(s2) > len(s1):
              raise ValueError("s1 must be larger than s2")
          if mask.shape != (len(s2), len(s1)):
              raise ValueError("mask has incorrect shape")
  
          return is_coord_subset_pbc(s2, s1, frac_tol, mask)
docs: Returns true if a matching exists between s2 and s2
          under frac_tol. s2 should be a subset of s1