Module CheckSingleton {
    \\ singleton
    \\ pointers and static groups are the same object because
    \\ each one has a pointer to same state (a tuple)
    \\ but from outside we do the miracle to have a static group to act as a pointer
    \\ We need a lambda function to hold the pointer to Singleton as closure
    Global One=lambda M=pointer() (aValue=0)-> {
        If M is type null then
            \\ one time happen
            Group Singleton {
            Type:One
            Private:
                state=(aValue,)
            Public:
                module Add (x) {
                    .state+=x
                }
                Set {Drop}
                Value {
                    =.state#val(0)
                }
            }
            M->group(Singleton)
        end if
        \\ return M which is a pointer
        =M
    }
    K=One(100)
    Print Eval(K)=100
    M=One()
    Print Eval(M)=100
    Print K is M = true
    Print K is type One = true
    K=>add 500
    Print eval(K)=600
    \\ copy K to Z (no pointer to Z, Z is named group)
    Z=Group(K)
    Print eval(z)=600, z=600
    Z.add 1000
    Print Z=1600, Eval(M)=1600, Eval(K)=1600
    \\ push a copy of Z, but state is pointer so we get a copy of a pointer
    Push Group(Z)
    Read beta
    Beta.add 1000
    Print Z=2600, Eval(M)=2600, Eval(K)=2600
    \\ convert pointer to group (a copy of group)
    group delta=One()
    delta.add 1000
    Print Z=3600, beta=3600, delta=3600, Eval(M)=3600, Eval(K)=3600
    \\ M and K are pointers to groups
    M=>add 400
    Print Z=4000, beta=4000, delta=4000, Eval(M)=4000, Eval(K)=4000
}
CheckSingleton