<\body> All user defined macros and style variables are stored in the ``current typesetting environment''. This environment associates a tree value to each string variable. Variables whose values are macros correspond to new primitives. The others are ordinary environment variables. The primitives for operating on the environment are available from . You may permanently change the value of an environment variable using the primitive, as in the example <\tm-fragment> >> You may also locally change the values of one or several environment variables using the primitive: <\tm-fragment> > The value of an environment variable may be retrieved using the primitive. This may for instance be used in order to increase a counter: <\tm-fragment> |1>>> Finally, you may associate logical properties to environment variables using the primitive. This is explained in more detail in the section about . <\initial> <\collection>