Just a list of constants defined with a name, a definition
and a value. They can be modified by the user.
Of course the modification must be consistent with the role of
the constant, and only values should be modified.
It is a named list, one component for each constant. A sublist is associated to each constant with two components: $d for the definition and $v for the value. Be aware that the value can be any object (vector, list, matrix, function,...)
A list (it is a self-documented object).
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