A general output from GiNaC (<https://www.ginac.de/>) of harmonic polylogarithms is `H(a,b,c...,x)`. We want to convert it to Mathematica format (<https://www.physik.uzh.ch/data/HPL/>) `HPL[{a,b,c...},x]`.

Note: we need "a general string expression" for the "a,b,c..." in
`H(a,b,c...,x)` as we have to deal with a lot of `H`s, for example, converting:

```
H(2,2,x),H(2,1,x),H(2,1,2,x),H(1,2,3,4,x),H(1,2,3,4,5,x)

```

to

```
HPL[{2,2},x],HPL[{1,2},x],HPL[{1,2,3},x],HPL[{1,2,3,4},x],HPL[{1,2,3,4,5},x]

```