code: public static IfNotExistsFunction<BOOL> if_not_exists(String path,
              boolean defaultValue) {
          return if_not_exists(new PathOperand(path), new LiteralOperand(
                  defaultValue));
      }
docs: Returns an <code>IfNotExists</code> object which represents an <a href=
  "http://docs.aws.amazon.com/amazondynamodb/latest/developerguide/Expressions.Modifying.html"
  >if_not_exists(path, operand)</a> function call; used for building
  expression.
  
  <pre>
  "if_not_exists (path, operand) – If the item does not contain an attribute
  at the specified path, then if_not_exists evaluates to operand; otherwise,
  it evaluates to path. You can use this function to avoid overwriting an
  attribute already present in the item."
  </pre>
  
  @param path
  document path to an attribute
  @param defaultValue
  default value if the attribute doesn't exist
  
  @return an <code>IfNotExists</code> object for boolean (BOOL) attribute.