Syntax:  DELETE TAB

  Erases blanks and tabs to the left of the cursor, which moves the
  cursor to the previous tab stop.

  This command moves the cursor to the previous tab stop set by the SET
  BUFFER TAB INCREMENT command.  If no tabs or blanks immediately
  precede the cursor, this command has no effect.  If nonblank or
  nontab characters are present in the column positions at or after the
  previous tab stop, LSE removes the blanks and tabs between those
  characters and the cursor, and then repositions the cursor after
  those characters, but not at the tab stop.