I was having fun with this as well while Hans was writing his generic solution.

Here is an explicit ‘indent each 2nd and 3rd line’:

%%%%%%%%
\unprotect

\installindentingmethod {three} {\c_spac_indentation_toggle_state\plusthree}

\unexpanded\def\spac_indentation_tri_toggle_indeed
  {\glet\checkindentation\spac_indentation_tri_toggle}

\unexpanded\def\spac_indentation_tri_toggle
  {\gdef\checkindentation
      {\spac_indentation_remove
       \gdef\checkindentation{\spac_indentation_tri_toggle_indeed}}}

% this next macro is redefinition of a core macro !
\def\spac_indentation_check_toggle
 {\ifcase\c_spac_indentation_toggle_state
    % nothing
  \or
    \spac_indentation_no_toggle
  \or
    \spac_indentation_do_toggle
  \or
    \spac_indentation_tri_toggle
  \fi}
\protect
%%%%%%%%%%

to be used with:

  \setuplines[indenting=three]


Best wishes,
Taco

> On 2 Jul 2020, at 15:01, Tommaso Gordini <illinguista1972@gmail.com> wrote:
>
> \starttext
>
> \setupindenting[yes,1em]
> \setuplines[indenting=even]
>
> \startlines
> Per correr miglior acque alza le vele
> omai la navicella del mio ingegno,
> che lascia dietro a sé mar sì crudele;
> e canterò di quel secondo regno
> dove l'umano spirito si purga
> e di salire al ciel diventa degno.
> \stoplines
>
> \stoptext

Taco Hoekwater
Elvenkind BV




___________________________________________________________________________________
If your question is of interest to others as well, please add an entry to the Wiki!

maillist : ntg-context@ntg.nl / http://www.ntg.nl/mailman/listinfo/ntg-context
webpage  : http://www.pragma-ade.nl / http://context.aanhet.net
archive  : https://bitbucket.org/phg/context-mirror/commits/
wiki     : http://contextgarden.net
___________________________________________________________________________________