There are several types of numbers
where a subset of numbers can be considered as set of scalars.
 A '(Complex.T Double)' value can be converted to Double if the imaginary part is zero.
 A value with physical units can be converted to a scalar if there is no unit.
Of course this can be cascaded,
e.g. a complex number with physical units can be converted to a scalar
if there is both no imaginary part and no unit.
This is somewhat similar to the multitype classes NormedMax.C and friends.
I hesitate to define an instance for lists
to avoid the mess known of MatLab.
But if you have an application where you think
you need this instance definitely
I'll think about that, again.
