diff --git a/bin/extract_tests.xsh b/bin/extract_tests.xsh
index 1d8c59f..422acb1 100644
--- a/bin/extract_tests.xsh
+++ b/bin/extract_tests.xsh
@@ -13,6 +13,9 @@ DESTINATION = 'tests/extracted'
 # Extension for extracted files:
 EXT = 'fos'
 
+# Extension for desired input:
+INP = 'in'
+
 # Extension for expectations:
 EXP = 'expect'
 
@@ -34,6 +37,11 @@ for path in TEST_LIST:
     expath = destdir / f"{name}.{EXT}"
     expath.write_text(example)
     echo Wrote @(expath)
+    im = re.search(r'/\*\*\s+accepts.*?\n([\s\S]*?)\*\*/', details[em.end():])
+    if im:
+      ipath = destdir / f"{name}.{INP}"
+      ipath.write_text(im[1])
+      echo "  ...and" @(ipath)
     xm = re.search(r'/\*\*\s+writes.*?\n([\s\S]*?)\*\*/', details[em.end():])
     if xm:
       xpath = destdir / f"{name}.{EXP}"
diff --git a/bin/run_tests b/bin/run_tests
index 90ccaa7..da49485 100755
--- a/bin/run_tests
+++ b/bin/run_tests
@@ -9,8 +9,14 @@ diffed=0
 for dir in tests/extracted/*; do
   for file in $dir/*.$ext; do
     ((total++))
-    $command $file > $file.out
-    if [[ $? -ne 0 ]]; then
+    if [[ -f ${file%.*}.in ]]; then
+       cat ${file%.*}.in | $command $file > $file.out
+       result=$?
+    else
+       $command $file > $file.out
+       result=$?
+    fi
+    if [[ $result -ne 0 ]]; then
       echo ERROR: $command $file failed.
       ((failed++))
     else
diff --git a/mkdocs.yml b/mkdocs.yml
index e316a98..c2d5242 100644
--- a/mkdocs.yml
+++ b/mkdocs.yml
@@ -9,7 +9,7 @@ plugins:
 - search
 - semiliterate:
     ignore_folders: [target, lib]
-    exclude_extensions: ['.o', '.hi']
+    exclude_extensions: ['.o', '.hi', '.cmi', '.cmo']
     extract_standard_markdown:
       terminate: <!-- /md -->
 theme:
diff --git a/syntax/fostr.sdf3 b/syntax/fostr.sdf3
index 8624b58..f782466 100644
--- a/syntax/fostr.sdf3
+++ b/syntax/fostr.sdf3
@@ -42,10 +42,13 @@ context-free syntax
   Ex.EscString = STRING
   Ex.Stream    = <stream>
   Ex.Sum       = <<Ex> +  <Ex>> {left}
+  Ex.Concat    = <<Ex> ++ <Ex>> {left}
   Ex.Gets      = [[Ex] << [Ex]] {left}
   Ex.DefGets   = [<<< [Ex]]
   Ex.To        = [[Ex] >> [Ex]] {left}
   Ex.DefTo     = [[Ex] >>>]
+  Ex.Emits     = <<Ex>!>
+  Ex.DefEmits  = <!!>
 
   Ex           = <(<Ex>)> {bracket}
 
@@ -53,7 +56,7 @@ context-free priorities
 
   Ex.To
   > Ex.DefTo
-  > Ex.Sum
+  > {Ex.Sum Ex.Concat}
   > Ex.DefGets
   > Ex.Gets,
 
diff --git a/tests/basic.spt b/tests/basic.spt
index a5e98c0..3d5a4a6 100644
--- a/tests/basic.spt
+++ b/tests/basic.spt
@@ -293,4 +293,38 @@ test emit_several_default [[
                 >>>
 ]] parse succeeds
 /** writes
-3399677527121313*/
+3399677527121313**/
+
+/** md
+### Streams are bidirectional
+
+So far we have only sent items to a stream. But we can extract them from
+streams as well, with the `!` postfix operator. `!!` all by itself abbreviates
+`stream!`, i.e., extraction from the standard stream. For example,
+
+```fostr
+**/
+
+/** md */ test custom_hw [[
+<<< "What is your name?\n"
+<<< 'Hello, ' ++ !!
+]] /* **/
+parse to TopLevel(Sequence([
+  DefGets(EscString("\"What is your name?\n\"")),
+  DefGets(Concat(LitString("'Hello, '"),DefEmits()))
+]))
+/** accepts
+Kilroy
+**/
+/** writes
+What is your name?
+Hello, Kilroy
+**/
+
+/** md
+```
+
+queries users for their name and then writes a customized greeting. It also
+illustrates the use of `++` for string concatenation, as opposed to `+` for
+(numerical) addition.
+**/
diff --git a/trans/desugar.str b/trans/desugar.str
index 7276f38..b22ebd8 100644
--- a/trans/desugar.str
+++ b/trans/desugar.str
@@ -17,6 +17,7 @@ rules
   
   defStream:  DefGets(x)  -> Gets(Stream(), x)
   defStream:  DefTo(x)    -> To(x, Stream())
+  defStream:  DefEmits()  -> Emits(Stream())
 
 strategies
 
diff --git a/trans/haskell.str b/trans/haskell.str
index 0b08716..0e0eb15 100644
--- a/trans/haskell.str
+++ b/trans/haskell.str
@@ -23,6 +23,7 @@ rules
   import System.IO
   data IOStream = StdIO
 
+  -- Danger: These currently assume the stream is StdIO
   gets :: Show b => a -> b -> IO a
   gets s d = do
     putStr(show d)
@@ -33,6 +34,10 @@ rules
     putStr(d)
     return s
 
+  emit s = do
+    l <- getLine
+    return (l ++ "\n")
+
   main = do
     [<Preactions>()]return [val]]
 
@@ -41,6 +46,7 @@ rules
   hs: (_, LitString(x)) -> <haskLitString>x
   hs: (_, EscString(x)) -> x
   hs: (_, Sum(x, y))    -> $[([x] + [y])]
+  hs: (_, Concat(x, y)) -> $[([x] ++ [y])]
 
   hs: (Gets(_, xn), Gets(s, x)) -> v
     with v := <newname>"_fostr_get"
@@ -52,6 +58,10 @@ rules
   hs_gets: (s, xn, x ) -> $[[s] [<hs_getOp>xn] [x]]
   hs_getOp = get-type; (?STRING() < !"`getsStr`" + !"`gets`")
 
+  hs: (_, Emits(s)) -> v
+    with v := <newname>"_fostr_emitted"
+       ; <add-preactions>[$[[v] <- emit [s]]]
+
   hs: (_, Terminate(x)) -> $[[x];;]
   hs: (_, Sequence(l))  -> <last>l
   /* One drawback of using paramorphism is we have to handle lists
diff --git a/trans/javascript.str b/trans/javascript.str
index 3a847e2..9b596ad 100644
--- a/trans/javascript.str
+++ b/trans/javascript.str
@@ -3,24 +3,41 @@ imports libstrategolib signatures/- util
 
 rules
   js: TopLevel(x) -> $[// Fostr preamble
+  const _fostr_readline = require('readline');
+  const _fostr_events = require('events');
+  const _fostr_rl = _fostr_readline.createInterface({input: process.stdin});
   const Stdio = {
-    gets: v => { process.stdout.write(String(v)); return Stdio; },
+    gets:  v => { process.stdout.write(String(v)); return Stdio; },
+    emit: async () => {
+            const [line] = await _fostr_events.once(_fostr_rl, 'line');
+            return line + "\n"; }
   }
   function to(data, strm) {
     strm.gets(data);
     return data;
   }
+
+  const _fostr_body = async () => {
   // End of preamble
 
-  [x]]
+    [x]
+
+  // Fostr coda
+    _fostr_rl.close()
+  }
+  _fostr_body();
+  ]
+    with line := "[line]"
 
   js: Stream()     -> $[Stdio]
   js: Int(x)       -> x
   js: LitString(x) -> <javaLitString>x
   js: EscString(x) -> x
-  js: Sum(x,y)     -> $[[x] + [y]]
+  js: Sum(x, y)    -> $[[x] + [y]]
+  js: Concat(x, y) -> $[[x] + [y]]
   js: Gets(x, y)   -> $[[x].gets([y])]
   js: To(x, y)     -> $[to([x],[y])]
+  js: Emits(x)     -> $[(await [x].emit())]
   js: Terminate(x) -> x
   js: Sequence(l)  -> <join(|";\n")>l
 
diff --git a/trans/ocaml.str b/trans/ocaml.str
index 11fcdca..049dbf4 100644
--- a/trans/ocaml.str
+++ b/trans/ocaml.str
@@ -12,9 +12,10 @@ imports libstrategolib signatures/- util signature/TYPE analysis
 
 rules
   ml: (_, TopLevel(x)) -> $[(* fostr preamble *)
-  type stream = { getS: string -> stream }
+  type stream = { getS: string -> stream; emitS: unit -> string }
   let rec stdio = {
-    getS = (fun s -> print_string s; stdio)
+    getS = (fun s -> print_string s; stdio);
+    emitS = (fun () -> (read_line ()) ^ "\n");
   };;
   (* End of preamble *)
 
@@ -24,11 +25,15 @@ rules
   ml: (_, Int(x))       -> x
   ml: (_, LitString(x)) -> $[{|[<un-single-quote>x]|}]
   ml: (_, EscString(x)) -> x
-  ml: (_, Sum(x,y))     -> $[[x] + [y]]
+  ml: (_, Sum(x, y))    -> $[[x] + [y]]
+  ml: (_, Concat(x, y)) -> $[[x] ^ [y]]
+
   ml: (Gets(_,yn), Gets(x, y))
     -> $[([x]).getS ([<ml_str>(yn,y)])]
   ml: (To(xn,_),   To(x, y))
     -> $[let _fto = ([x]) in (ignore (([y]).getS ([<ml_str>(xn,"_fto")])); _fto)]
+  ml: (_, Emits(s))     -> $[[s].emitS ()]
+
   ml: (_, Terminate(x)) -> x
   ml: (_, Sequence(l))  -> <ml_seq>l
   
diff --git a/trans/python.str b/trans/python.str
index 6449417..f42b1cb 100644
--- a/trans/python.str
+++ b/trans/python.str
@@ -8,6 +8,8 @@ rules
     def gets(self, v):
       print(v, file=sys.stdout, end='')
       return self
+    def emit(self):
+      return input() + "\n" # Python inconsistently strips when using input
   def to(data,strm):
     strm.gets(data)
     return data
@@ -21,8 +23,10 @@ rules
   py: LitString(x) -> $[r[x]]
   py: EscString(x) -> x
   py: Sum(x,y)     -> $[[x] + [y]]
+  py: Concat(x,y)  -> $[[x] + [y]]
   py: Gets(x, y)   -> $[[x].gets([y])]
   py: To(x, y)     -> $[to([x],[y])]
+  py: Emits(x)     -> $[[x].emit()]
   py: Terminate(x) -> $[[x];]
   py: Sequence(l)  -> <join(|"\n")>l
 
diff --git a/trans/statics.stx b/trans/statics.stx
index 340a1df..ce99bf7 100644
--- a/trans/statics.stx
+++ b/trans/statics.stx
@@ -222,6 +222,13 @@ This pattern lets us specify error messages.
     type_Ex(e2) == STREAM() | error $[Items may only be sent to Streams.]@e2.
   /* **/
 
+  ty_Ex(Concat(e1, e2)) = STRING() :-
+    type_Ex(e1) == STRING() | error $[Expression [e1] not String in concat.]@e1,
+    type_Ex(e2) == STRING() | error $[Expression [e2] not String in concat.]@e2.
+
+  ty_Ex(Emits(e)) = STRING() :- // At the moment, only stream is stdio
+    type_Ex(e) == STREAM() | error $[Only Streams may emit items.]@e.
+
   /** md
 ```