anchor.diff:  evaluation context, root object and any predefined variables are all set up implicitly,
   requiring the user to specify nothing other than the expressions.
   ====
  +
   As a final introductory example, the use of a boolean operator is shown using the
   Inventor object in the previous example.
   
  
positive.code: 	// evaluates to true
  	boolean trueValue = parser.parseExpression("'black' < 'block'").getValue(Boolean.class);
  ----
  
  In addition to standard relational operators SpEL supports the `instanceof` and regular
  expression based `matches` operator.
  
negative.code: 
  [[expressions-ref-literal]]
  === Literal expressions
  The types of literal expressions supported are strings, numeric values (int, real, hex),
  boolean and null. Strings are delimited by single quotes. To put a single quote itself
  in a string, use two single quote characters.
  
  The following listing shows simple usage of literals. Typically they would not be used
  in isolation like this but rather as part of a more complex expression, for example
  using a literal on one side of a logical comparison operator.
  
  [source,java,indent=0]
  [subs="verbatim,quotes"]
  