Pricing and charging arrangements available to communication network operators or service providers take account of factors such as taxes which affect the billable value and which may be put in place in many different ways and at variable times in relation to data processing exercises which need to be carried out. A system is provided which can be used in generating bills by applying modifying factors which can be changed or added to in a relatively easy manner. An object-oriented front end for a legacy system can be used in design and subsequently embedded. Programs developed in an object oriented environment are converted into instructions implementable by a procedurally based processing device. An activity hierarchy of functions is created for each object, a data subject area is created for each object and a main calling program specifies the relationship between the programs and data areas so as to retain encapsulation of the original object oriented definition. This is particularly useful for developing new systems in an object oriented environment which may then be embedded into existing procedurally based systems.