code: public Variable getArgument(int index) {
          Validate.isTrue(index >= 0 && index < argVars.size());
          return argVars.get(index);
      }
docs: Get the variable for an argument passed in to the method. Remember that if the method is static, arguments start at 0th index in the
  local variables table. If it isn't static, they start at 1 (0th would be "this").
  <p>
  Values returned by this method must never be passed in to
  {@link #releaseExtra(com.offbynull.coroutines.instrumenter.asm.VariableTable.Variable) }.
  @param index index of argument
  @return {@link Variable} object that represents the argument
  @throws IllegalArgumentException if {@code index} is not the index of an argument