+
+/* Decompose FILENAME in four parts: *BASE, *TAB, and *EXT, the fourth
+ part, (the directory) is ranging from FILENAME to the char before
+ *BASE, so we don't need an additional parameter.
+
+ *EXT points to the last period in the basename, or NULL if none.
+
+ If there is no *EXT, *TAB is NULL. Otherwise, *TAB points to
+ `.tab' or `_tab' if present right before *EXT, or is NULL. *TAB
+ cannot be equal to *BASE.
+
+ None are allocated, they are simply pointers to parts of FILENAME.
+ Examples:
+
+ '/tmp/foo.tab.c' -> *BASE = 'foo.tab.c', *TAB = '.tab.c', *EXT =
+ '.c'
+
+ 'foo.c' -> *BASE = 'foo.c', *TAB = NULL, *EXT = '.c'
+
+ 'tab.c' -> *BASE = 'tab.c', *TAB = NULL, *EXT = '.c'
+
+ '.tab.c' -> *BASE = '.tab.c', *TAB = NULL, *EXT = '.c'
+
+ 'foo.tab' -> *BASE = 'foo.tab', *TAB = NULL, *EXT = '.tab'
+
+ 'foo_tab' -> *BASE = 'foo_tab', *TAB = NULL, *EXT = NULL
+
+ 'foo' -> *BASE = 'foo', *TAB = NULL, *EXT = NULL. */
+