feat WIP: checkpoint initial efforts on stream extraction

Adds ! operator and !! expression, with Python code gen and
  an item in tour.
  Still needs: other languages, check generated docs, and addition
  of standard input to test scheme.

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
 

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()
   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
 

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
 ```