Note . . . Some functions have no symbolic derivative.
The only alternative is infinitesimal from the CAS ancestors.
In this example,'h' is paired with erf. In common Engineering
functions, too low 'h' will generate noise. Infinitesimal is the
sole Smath method to get the derivative of splined functions.
Differently, Mathcad has an equivalent scalar method, about
equivalent to infinitesimal.
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