Enumerate all defined variables with their value. The order of
enumeration is undefined. Note that nb_current/2
can be used as an alternative for nb_getval/2
to request the value of a variable and fail silently if the variable
does not exists. Note that if the variable is not defined, exception/3
is called attempting to define it. As of version 8.3.28, a failure of exception/3
to define the variable causes the variable to be defined with a reserved
valued to avoid subsequent calls to exception/3.