docs: def convert_sub(sub):
      """Convert BEL1 sub() to BEL2 var()"""
  
      args = sub.args
      (ref_aa, pos, new_aa) = args
  
      parent_fn_name = sub.parent_function.name_short
      prefix_list = {"p": "p.", "r": "r.", "g": "c."}
      prefix = prefix_list[parent_fn_name]
  
      new_var_arg = f'"{prefix}{spec["namespaces"]["AminoAcid"]["to_short"][ref_aa.value]}{pos.value}{spec["namespaces"]["AminoAcid"]["to_short"][new_aa.value]}"'
  
      new_var = bel.lang.ast.Function("var", bo.spec)
  
      new_var.add_argument(StrArg(new_var_arg, new_var))
  
      return new_var