The Li function is defined in Maple as
> FunctionAdvisor(Li, definition)
[Li(z) = Ei(ln(z)), with no restrictions on (z)]
Special values are known
> FunctionAdvisor(Li, special_values)
Warning: when function identities information is required, only one argument -
the function name - is expected. Extra arguments are being ignored.
[Li(0) = 0, Li(1) = -infinity, Li(infinity) = infinity]
> Li(0), Li(infinity)
Error, (in Li) numeric exception: division by zero
This is consistent with a computation of Ei(ln(1))=Ei(0) but,
why the Li(1)=-infinity is not in the Cache or remember table of Li ?
PS : I know I can put myself the value in the remember table
> Li(1) := -infinity