underscore in BibTeX keys means trouble: do not use it

duplicate words removed from various documentation files

The magic command is:

grep " \([[:alpha:]]\+\) \1[[:space:]]" \
`find . -name "*.doc" -or -name "*.tex"`

but the output still needs to be filtered by eye, since many duplicates
occur in embedded math and ML.

Duplicates in the Italian manual approved by Domenico Masini.

Minor corrections to the tutorial, and half translation of description.