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diff --git a/Tools/LuaMacro/macro/do.lua b/Tools/LuaMacro/macro/do.lua
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+--- An intelligent 'loop-unrolling' macro.
+-- `do_` defines a named scoped macro `var` which is the loop iterator.
+--
+-- For example,
+--
+--    y = 0
+--    do_(i,1,10
+--       y = y + i
+--    )
+--    assert(y == 55)
+--
+-- `tuple` is an example of how the expansion of a macro can be
+-- controlled by its context. Normally a tuple `A` expands to
+-- `A_1,A_2,A_3` but inside `do_` it works element-wise:
+--
+--    tuple(3) A,B
+--    def_ do3(stmt) do_(k,1,3,stmt)
+--    do3(A = B/2)
+--
+--  This expands as
+--
+--    A_1 = B_1/2
+--    A_2 = B_2/2
+--    A_3 = B_3/2
+--
+-- @module macro.do
+local M = require 'macro'
+
+--- Expand a loop inline.
+-- @p var the loop variable
+-- @p start initial value of `var`
+-- @p finish final value of `var`
+-- @p stat the statement containing `var`
+-- @macro do_
+M.define('do_(v,s,f,stat)',function(var,start,finish,statements)
+    -- macros with specified formal args have to make their own putter,
+    -- and convert the actual arguments to the type they expect.
+    local put = M.Putter()
+    var,start,finish = var:get_iden(),start:get_number(),finish:get_number()
+    M.push_macro_stack('do_',var)
+    -- 'do_' works by setting the variable macro for each value
+    for i = start, finish do
+        put:name 'set_':name(var):number(i):space()
+        put:tokens(statements)
+    end
+    put:name 'undef_':name(var)
+    put:name '_DROP_':string 'do_':space()
+    return put
+end)
+
+--- an example of conditional expansion.
+-- `tuple` takes a list of variable names, like a declaration list except that it
+-- must end with a line end.
+-- @macro tuple
+M.define('tuple',function(get)
+    get:expecting '('
+    local N = get:number()
+    get:expecting ')'
+    local names = get:names '\n'
+    for _,name in ipairs(names) do
+        M.define(name,function(get,put)
+            local loop_var = M.value_of_macro_stack 'do_'
+            if loop_var then
+                local loop_idx = tonumber(M.get_macro_value(loop_var))
+                return put:name (name..'_'..loop_idx)
+            else
+                local out = {}
+                for i = 1,N do
+                    out[i] = name..'_'..i
+                end
+                return put:names(out)
+            end
+        end)
+    end
+end)