Copy the structure of the existing lines indent when autoindenting a new line. Normally the new indent is reconstructed by a series of tabs followed by spaces as required (unless 'expandtab' is enabled, in which case only spaces are used). Enabling this option makes the new line copy whatever characters were used for indenting on the existing line. 'expandtab' has no effect on these characters, a Tab remains a Tab. If the new indent is greater than on the existing line, the remaining space is filled in the normal manner. NOTE: This option is reset when 'compatible' is set. Also see 'preserveindent'.

(default off)