feat: Allow expressions to be terminated/sequenced by ;

Note that ultimately a terminated sequence may have
   a slightly different semantics (applying streams
   to `_|_`, most likely) but for now they don't.

syntax/fostr.sdf3

@@ -10,16 +10,24 @@ context-free start-symbols
 
 context-free sorts
 
-  Start LineSeq Line Ex
+  Start LineSeq Line OptTermEx TermExLst TermEx Ex
 
 context-free syntax
   
   Start.TopLevel   = LineSeq
 
-  LineSeq          = <<ln:Ex>> {layout(offside ln)}
-  LineSeq.Sequence = sq:Ex+    {layout(align-list sq)}
+  LineSeq          = Line
+  LineSeq.Sequence = sq:Line+            {layout(align-list sq)}
 
-  Ex+         = Ex+ ln:Ex {layout(offside ln)}
+  Line             = OptTermEx
+  Line.ISequence   = TermExLst OptTermEx {layout(0.first.line == 1.first.line)}
+
+  TermExLst.Prior  = TermEx+
+
+  OptTermEx        = ex:Ex               {layout(offside ex)}
+  OptTermEx        = te:TermEx           {layout(offside te)}
+
+  TermEx.Terminate = <<Ex>;>
 
   Ex.Int      = INT
   Ex.Stream   = <stream>
@@ -36,8 +44,4 @@ context-free priorities
   > Ex.Gets,
 
   // prevent cycle: no singletons
-  LineSeq.Sequence <0> .> Ex+       = Ex,
-
-  // flat: No LineSeq immediately in LineSeq
-  Ex+ = Ex <0>     .> LineSeq.Sequence,
-  Ex+ = Ex+ Ex <1> .> LineSeq.Sequence
+  LineSeq.Sequence <0> .> Line+ = Line