code: def base0_interval_for_variant_fields(base1_location, ref, alt):
      
      if len(ref) == 0:
          # in interbase coordinates, the insertion happens
          # at the same start/end offsets, since those are already between
          # reference bases. Furthermore, since the convention for base-1
          # coordinates is to locate the insertion *after* the position,
          # in this case the interbase and base-1 positions coincide.
          base0_start = base1_location
          base0_end = base1_location
      else:
          # substitution or deletion
          base0_start = base1_location - 1
          base0_end = base0_start + len(ref)
      return base0_start, base0_end
docs: Inteval of interbase offsets of the affected reference positions for a
      particular variant's primary fields (pos, ref, alt).
  
      Parameters
      ----------
      base1_location : int
          First reference nucleotide of variant or, for insertions, the base
          before the insertion.
  
      ref : str
          Reference nucleotides
  
      alt : str
          Alternative nucleotides