As explained in section 2, the vocabulary of a logical language can be extended by definitions of new concepts in terms of primitives and previously defined concepts. This extensibility can easily be provided by program declarations such as the following:
In most cases it will be convenient to supply the user with a number of useful predefined non-primitive concepts along with a decision procedure. These will have a status similar to standard library functions in C. Additional libraries of more complex defined concepts might also be created for specialised use of a DP. (As with libraries for other programming languages it may sometimes be desirable to optimise defined concepts by implementing them not as logical definitions but by means of lower level code.)