@@ 1,4 1,7 @@ 
-                         
+
+(** A file to which vectors can be written and read. All indexing is
+    done in units of the vector's element size.
+ *)
 signature BACKING_FILE_BASE = sig
 
     type file

          
@@ 40,6 43,12 @@ signature TEMP_BACKING_FILE = sig
                         
 end
 
+(** BACKED_VECTOR stores a vector to some representation (nominally a
+    file) and allows the whole thing or individual values to be read
+    back again. The file type is not constrained in the signature, but
+    the file is specified, because implementations may allow storing
+    more than one vector in the same file.
+ *)
 signature BACKED_VECTOR = sig
 
     type elem