XL has a method to extend the language with any kind of feature, not just functions or data types, but also programming constructs, optimizations, domain-specific notations, and more. Actually, all this is done with a single operator, is, called the definition operator. As a validation of the concept, most features that are built-in in other programming languages, like the while loop, or integer arithmetic, are constructed in XL. Specifically, they are provided by the standard library, using techniques that any programmer can use in their program. This, obviously, means that programmers can add their own loops, or their own machine-level data types, and even extend existing ones. XL provides complete control over the program translation process. This means that libraries exist or can be written to make XL at least as good as C for low-level bit-twiddling, at least as good as C++ for generic algorithms, at least as good as Ada for tasking, at least as good as Fortran for numerical algorithms, at least as good as Java for distributed programming, and so on.