How to merge Headers of index and other headers to one row of headers and add numeric index as single index [duplicate]

I currently have this type of a df with 2 headers

        client case  client payment  payment numberclient  foo            1             200               1boo            1             250               2moo            1             225               3

and I would like a numeric index and merge the header into one single header and for the previous index to be the first column:

  client  client case  client payment  payment number0  foo            1             200               11  boo            1             250               22  boo            1             225               3

df.columns = [c[0] + c[1] for c in df.columns]this did not work for me, nor dit several set_index options