code: public static CompoundAssignment compoundAssignment(
        Kind operator,
        Matcher<ExpressionTree> leftOperandMatcher,
        Matcher<ExpressionTree> rightOperandMatcher) {
      Set<Kind> operators = new HashSet<>(1);
      operators.add(operator);
      return new CompoundAssignment(operators, leftOperandMatcher, rightOperandMatcher);
    }
docs: Matches a compound assignment operator AST node which matches a given left-operand matcher, a
  given right-operand matcher, and a specific compound assignment operator.
  
  @param operator Which compound assignment operator to match against.
  @param leftOperandMatcher The matcher to apply to the left operand.
  @param rightOperandMatcher The matcher to apply to the right operand.