the
name of the column that provides a unique ID for every trial (the trial
identifier). If unspecified (the default), an ID variable will be
generated. If more than one variable name is provided, a new ID variable
will be created by combining the values of each variable. The trial
identifier will be set as the <a href="https://rdrr.io/r/base/colnames.html" class="external-link">rownames</a> of the resulting trajectories
and trial data, and additionally be stored in the column "mt_id" in the
trial data.</p>