Function parse_add_plan 

pub fn parse_add_plan(
    arguments: &[OsString],
    subcommand_index: usize,
    start: &Path,
) -> Option<AddPlan>

What: Plan one invocation: arguments is the whole argument list, subcommand_index the index of worktree in it, and start the directory the command starts in. Why: The word directly after worktree must be add itself, because Git reads the subcommand there and never abbreviates it.

In TS you’d write (pseudocode):

function parseAddPlan(args: string[], subcommandIndex: number, start: string): AddPlan | undefined;