{not available when compiled without the |+eval| feature} This option specifies a function to be used for Insert mode omni completion with CTRL-X CTRL-O. |i_CTRL-X_CTRL-O| See |complete-functions| for an explanation of how the function is invoked and what it should return. This option is usually set by a filetype plugin: |:filetype-plugin-on| This option cannot be set from a |modeline| or in the |sandbox|, for security reasons.