hush: delete unused field in struct child.

reinstate needed check for invalid syntax.
 document command parsing in hush_doc.txt.

shell/hush_doc.txt

@@ -1,3 +1,80 @@
+2008-07-14
+
+	Command parsing
+
+Command parsing results in "pipe" structures. "Pipe" structure
+does not always correspond to what sh language calls "pipe",
+it also controls execution of if, while, etc statements.
+
+struct pipe fields:
+  smallint res_word - "none" for normal commands,
+                      "if" for if condition etc
+  struct child_prog progs[] - array of commands in pipe
+  smallint followup - how this pipe is related to next: is it
+                      "pipe; pipe", "pipe & pipe" "pipe && pipe",
+                      "pipe || pipe"?
+
+Blocks of commands { pipe; pipe; } and (pipe; pipe) are represented
+as one pipe struct with one progs[0] element which is a "group" -
+struct child_prog can contain a list of pipes. Sometimes these
+"groups" are created implicitly, e.g. every control
+statement (if, while, etc) sits inside its own "pipe" struct).
+
+res_word controls statement execution. Examples:
+
+"echo Hello" -
+pipe 0 res_word=NONE followup=SEQ prog[0] 'echo' 'Hello'
+pipe 1 res_word=NONE followup=1 SEQ
+
+"echo foo || echo bar" -
+pipe 0 res_word=NONE followup=OR  prog[0] 'echo' 'foo'
+pipe 1 res_word=NONE followup=SEQ prog[0] 'echo' 'bar'
+pipe 2 res_word=NONE followup=SEQ
+
+"if true; then echo Hello; true; fi" -
+res_word=NONE followup=SEQ
+ prog 0 group {}:
+  pipe 0 res_word=IF followup=SEQ prog[0] 'true'
+  pipe 1 res_word=THEN followup=SEQ prog[0] 'echo' 'Hello'
+  pipe 2 res_word=THEN followup=SEQ prog[0] 'true'
+  pipe 3 res_word=FI followup=SEQ
+  pipe 4 res_word=NONE followup=(null)
+pipe 1 res_word=NONE followup=SEQ
+
+"if true; then { echo Hello; true; }; fi" -
+pipe 0 res_word=NONE followup=SEQ
+ prog 0 group {}:
+  pipe 0 res_word=IF followup=SEQ prog[0] 'true'
+  pipe 1 res_word=THEN followup=SEQ
+   prog 0 group {}:
+    pipe 0 res_word=NONE followup=SEQ prog[0] 'echo' 'Hello'
+    pipe 1 res_word=NONE followup=SEQ prog[0] 'true'
+    pipe 2 res_word=NONE followup=SEQ
+  pipe 2 res_word=NONE followup=(null)
+pipe 1 res_word=NONE followup=1 SEQ
+
+"for v in a b; do echo $v; true; done" -
+pipe 0 res_word=NONE followup=SEQ
+ prog 0 group {}:
+  pipe 0 res_word=FOR followup=SEQ prog[0] 'v'
+  pipe 1 res_word=IN followup=SEQ prog[0] 'a' 'b'
+  pipe 2 res_word=DO followup=SEQ prog[0] 'echo' '$v'
+  pipe 3 res_word=DO followup=SEQ prog[0] 'true'
+  pipe 4 res_word=DONE followup=SEQ
+  pipe 5 res_word=NONE followup=(null)
+pipe 1 res_word=NONE followup=SEQ
+
+Note how "THEN" and "DO" does not just mark the first pipe,
+it "sticks" to all pipes in the body. This is used when
+hush executes parsed pipes.
+
+Dummy trailing pipes with no commands are artifacts of imperfect
+parsing algorithm - done_pipe() appends new pipe struct beforehand
+and last one ends up empty and unused.
+
+
+2008-01
+
 	This is how hush runs commands:
 
 /* callsite: process_command_subs */