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
 into the LaTeX document. You may use also the corresponding
 environments `pyexec` and `pyeval`.
 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
 A Python function `tex(s, *args)` may be called from Python snippets
 to generate some LaTeX code that will be inserted in the document
 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).
 instead of the snippet (and, in the case of `\pyv`, after its result).