Specifies the abstract base class for all conditional expression classes.
Conditional expressions model expressions evaluating to a boolean expression type.
Conditional expressions are modeled in 3 basic ways.
Using one of the SimpleConditionalExpression_* classes - these classes model one of the ten or so types of simple conditional expressions, as laid out
by SQL and shown below.
Using the CompoundConditionalExpression class - this class models the ability to construct
compound expressions with tied together with ANDs and ORs.
Using the StringExpression class - to catch all other conditional expressions not modeled in any other way.