If a function requires an optional package, that function should fail gracefully --
perhaps using a try and except block -- when the optional
package is not available, and should give a hint about how to install
it. For example, typing sage -optional gives a list of all
optional packages, so it might suggest to the user that they type that. The
command optional_packages() from within Sage also returns this list.
Release 2008.09.17, documentation updated on September 17, 2008.
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