Note: the input expression is the techincal expression (like
Expression Language) which extracts the value against which this clause
is tested from the input data.

TBA:

• Should the input expression always be visible? (Given that it is
  a "technical" property yet mandatory for execution?)
• How can I add a very "long" input expression? (Given that it may
  be any script...)
• How can can I define the expression / script language of the
  expression (Javascript, Groovy ...)