ANSI Common Lisp 22 Printer 22.3 Formatted Output 22.3.6 FORMAT Layout Control
188.8.131.52 Tilde T: Tabulate
This spaces over to a given column. ~colnum,colincT will output sufficient spaces to move the cursor to column colnum. If the cursor is already at or beyond column colnum, it will output spaces to move it to column colnum+k*colinc for the smallest positive integer k possible, unless colinc is zero, in which case no spaces are output if the cursor is already at or beyond column colnum. colnum and colinc default to 1.
If for some reason the current absolute column position cannot be determined by direct inquiry, format may be able to deduce the current column position by noting that certain directives (such as ~%, or ~&, or ~A with the argument being a string containing a newline) cause the column position to be reset to zero, and counting the number of characters emitted since that point. If that fails, format may attempt a similar deduction on the riskier assumption that the destination was at column zero when format was invoked. If even this heuristic fails or is implementationally inconvenient, at worst the ~T operation will simply output two spaces.
~@T performs relative tabulation. ~colrel,colinc@T outputs colrel spaces and then outputs the smallest non-negative number of additional spaces necessary to move the cursor to a column that is a multiple of colinc. For example, the directive ~3,8@T outputs three spaces and then moves the cursor to a "standard multiple-of-eight tab stop" if not at one already. If the current output column cannot be determined, however, then colinc is ignored, and exactly colrel spaces are output.
If the colon modifier is used with the ~T directive, the tabbing computation is done relative to the horizontal position where the section immediately containing the directive begins, rather than with respect to a horizontal position of zero. The numerical parameters are both interpreted as being in units of ems and both default to 1. ~n,m:T is the same as (pprint-tab :section n m). ~n,m:@T is the same as (pprint-tab :section-relative n m).