Create key for sorting

[lorenzwalthert]

line1/ col1 is no longer a unique sorting (composite) key because we insert tokens like with
add_brackets_in_pipe(). One way to go about that would be to setting col1/ col2 of these tokens
"in between" the existing tokens. New tokens could then get intermediate values (e.g., when () is added: key + 1/3 and key + 2/3 respectively).
See discussion in #111 R/visit.R L136+.

[krlmlr]

Reference: #111 (comment)

[lorenzwalthert]

Reference (#119).

[lorenzwalthert]

Maybe can simply use column id. Check with help file of getParseData().

[lorenzwalthert]

Reference: #147.