  This help file describes expresssions available in ${}, and the
  @, IF, WHILE, and FOREACH commands.

  Variable expressions are used to perform arithmetic, string and
  boolean operations. Their syntax is similar to the syntax of most
  computer languages, and is modelled on C syntax. A simple example
  of this, is that when ${A + 2} is used, and the ASSIGNed variable
  A has the value '3', the result is 5.

  Brackets can be used in these expressions, so (A+2)*3 would give a
  value of (3+2)*3 == 15, whereas A+2*3 would give a value of 9, because
  multiplication is performed before addition.

  The common arithmetic operations (+, -, *, /) are all available.
  Additionally, there is a string concatenation operator (##).
  This treats both sides as strings and pastes them together. Thus
  if A is 3 and B is 7, "A##B" becomes "37", but "A+B" becomes 10.

  You can also assign values to a variable in these expressions
  using the '=' operator. Thus if you have the expression "C = A+B",
  it will assign the value 10 to C. This has a number of affects.
  The most common case is the @ command, where you can enter this expression
  literally:

    @ C = A + B

  The '=' operator returns the value assigned. Thus you can extend
  this to:

    @ D = C = A + B

  Which will assign the value 10 to both C and D. In a $ expression,
  you might want to assign a value to a variable, and display it at the
  same time. Thus you might do the following:

    echo The value of C is now ${C = A+B}

  which would set C to 10 and display "The value of C is now 10".

  Comparison operations also exist. These return 1 if the comparison is
  valid (true) and 0 if it is invalid (false). The comparison operations
  available are:

    == (equal to)
    >  (greater than)
    >= (greater than or equal to)
    <  (less than)
    <= (less than or equal to)
    != (not equal to)

  Thus, the following expressions would result in the following results:
   ( recall A = 3 and B = 7 )

	A == B			FALSE	0
	A == 3			TRUE	1
	A > 3			FALSE	0
	A >= 3			TRUE	1
	A != 3			TRUE	1

  These expressions can be combined with || (OR), && (AND) and ^^ (XOR).
  So (A == 3) || (B==3) would be TRUE, and (A == 2) && (B == 7) would be
  FALSE. You can also negate expressions with ! (NOT), so !(A == 3) would
  be FALSE. These boolean expressions are used primarily in IF and WHILE.

  A string expression is considered to be true if it is non empty. So
  if E has the value "word", then E || ( A > 3) is true, because E has
  a non empty value, and !E is false.

  Functions can also appear in expressions. The arguments to functions
  are evaluated as normal $ type expressions. Thus the following alias:

    alias test echo ${ MID(3 2 $0) + 5) }

  is executed with /test 123456, would give  34 (Two digits from position 3
  in $0) + 5, giving 39. If the function returns another function name, that
  function name can be called with another set of brackets. Thus if you have
  the following set of aliases:

    alias fptr
    {
        if ( [$0] )
            { @ function_return = func1 }
            { @ function_return = func2 }
    }
    alias func1 @ function_return = this is $0
    alias func2 @ function_return = that is $0
    alias check echo ${ fptr($0)($1) }

  then /check 0 1 would print "that is 1", and /check 1 0 would print
  "this is 0".

  Array expressions can also be used in this way. For example:

    assign A.1.1 One One was a racehorse
    assign A.1.2 Two Two was one too
    assign A.2.1 One One won one race
    assign A.2.2 Two Two won one too
    alias rhyme echo ${ A[$0][$1] }

  would cause /rhyme 2 1 to print "One One won one race".

  Function and array expressions can be combined to give a form
  of pointer arithmetic like that seen in C. If a function
  RhymeNum exists as follows:

    alias RhymeNum @ function_return = [A]
  and rhyme is changed to:
    alias rhyme echo ${ RhymeNum()[$0][$1] }

  /rhyme 2 1 still prints "One One won one race". This is because
  RhymeNum returns A, and the expression then becomes A[$0][$1].
  The $0 is expanded to 2, so it becomes A.2[$1], and the $1 is
  expanded to 1, so it becomes A.2.1, which is then substituted as
  a variable for "One One won one race". The reverse is also possible.
  For example, a robot might have the following:

     alias thing.0 @ function_return = laughs his silly head off
     alias thing.1 @ function_return = growls menacingly
     alias thing.2 @ function_return = smiles like a crocodile
     alias thing.3 @ function_return = wails uncontrollably
     alias something SAY WereBot ${ thing[$RANDOM(4)]() }

  Will cause WereBot to say that it's laughing itself silly, growling
  menacingly, smiling like a crocodile or wailing uncontrollably.
  Assuming RANDOM(4) results in a value of 2, This expands to
  thing.2(), which is then considered to be a function and substituted
  to "smiles like a crocodile", the end result being that WereBot will
  say "WereBot smiles like a crocodile".

  Finally, there are occasions when you need to get back to the $
  substitution level. This can be done by enclosing text to be used at
  this level in [..]. For example, [A] substitutes to a literal 'A', 
  whereas A on its own substitutes to 3, and [$0] is needed to get
  argument 0, because 0 on its own is taken to be the number 0.
  Example:

    alias something SAY WereBot ${ [$0][$RANDOM(4)]() }

  will case /something thing to first expand [$0] to thing, giving
  thing[$RANDOM(4)](), and if $RANDOM(4) returns 1, this becomes
  thing.1(), which expands to "growls menacingly", and causes
  WereBot to say "WereBot growls menacingly".


  The following is the parse tree for expressions such as  those

	NU_EXPR = NU_CONJ
	NU_CONJ = NU_CONJ && NU_CONJ	|
		  NU_CONJ || NU_CONJ	|
		  NU_CONJ ^^ NU_CONJ	|
		  NU_ASSN
	NU_ASSN = varexp = NU_ASSN	|
		  NU_COMP
	NU_COMP = NU_COMP == NU_COMP	|
		  NU_COMP != NU_COMP	|
		  NU_COMP >  NU_COMP	|
		  NU_COMP >= NU_COMP	|
		  NU_COMP <  NU_COMP	|
		  NU_COMP <= NU_COMP	|
		  NU_ADD
	NU_ADD  = NU_ADD + NU_ADD	|
		  NU_ADD - NU_ADD	|
		  NU_ADD ## NU_ADD	|
		  NU_MULT
	NU_MULT = NU_MULT * NU_MULT	|
		  NU_MULT / NU_MULT	|
		  NU_UNIT
	NU_UNIT = token NUX_MODIF	|
		 unaryop token		|
		( NU_EXPR )		|
		[ expression ] NUX_MODIF

	NUX_MODIF = ( expression ) NUX_MODIF |
		    [ expression ] NUX_MODIF

Special cases:
  If a (...) or {...} construct is quoted with \(...\) or \{...\}
  then variable expansion will take place on the first available
  parse run.  However, after parsed, the \'s are eaten. e.g.
    alias bonk echo ($0)        will return "($*)" regardless of
  the argument to the alias.  No expansion takes place.  However, in
    alias bonk echo \($0\)
  expansion takes place as the special meaning of the () is taken
  away by \.  This will return "(arguments to bonk)".
