diff --git a/mtar/README.md b/mtar/README.md
index d0f6bd3..53cb015 100644
--- a/mtar/README.md
+++ b/mtar/README.md
@@ -12,3 +12,12 @@ mtar archive files consist of a number of file entries, in the following format:
 - the file contents
 
 Archives are terminated either by the end of file, or a record with a name length of zero.
+
+## libmtar
+
+libmtar is the reference implementation of mtar. It provides two functions:
+
+ - `libmtar.genHeader(string: name, number: len): string`  
+Generates an mtar file header for a file called *name* that is *len* bytes long.
+ - `libmtar.iter(table: stream): function`  
+Given buffer *stream*, returns an iterator suitable for use with *for* that returns, for each iteration, the file name, a function to read from the file, and the length of the file.
diff --git a/mtar/libmtar.lua b/mtar/libmtar.lua
index 75bf9e2..9c4adef 100644
--- a/mtar/libmtar.lua
+++ b/mtar/libmtar.lua
@@ -19,7 +19,7 @@ local function cint(n,l)
  return string.reverse(string.char(table.unpack(t)):sub(1,l))
 end
 
-function mtar.genHeader(fname,len) -- generate a header for file *fname* when provided with file length *len*
+function mtar.genHeader(fname,len) -- string number -- string -- Generates an mtar file header for a file called *name* that is *len* bytes long.
  return string.format("%s%s%s",cint(fname:len(),2),fname,cint(len,2))
 end