added doc abour reset()

@@ -33,6 +33,12 @@ should be passed to Python `eval` and the computed value integrated
 into the LaTeX document. You may use also the corresponding
 environments `pyexec` and `pyeval`.
 
+An important feature of `pytex` is that all the snippets are executed
+in the same environment, so that you can do something like `\pyx{x=5}
+balh blah \pyv{x+2} % yields 7`. If you wish to forget about all
+previous computation, use `\pyx{reset()}` which clears the execution
+environment.
+
 A Python function `tex(s, *args)` may be called from Python snippets
 to generate some LaTeX code that will be inserted in the document
 instead of the snippet (and, in the case of `\pyv`, after its result).