Provides utility functions to filter data and to create sql condition over database. Every function returns a function f where: f ( r, context ) = true if r matches condition in the given context f( r, context ) = result evaluated in the given context if f is a computation function f.isTrue = true if f is always true f.isFalse = true if f is always false f ( r, context) = undefined if there is no sufficient data to evaluate f null fields and undefined fields are all considered (and returned) as null values (so they compare equal) f.toSql(formatter, context) = a string representing the underlying condition to be applied to a database. formatter is used to obtain details about making the expression, see sqlFormatter for an example [context] is the context into which the expression have to be evaluated