CONTROL STRUCTURES
==================

Note: in the following descriptions, an "instructionlist" can be a list
or a word.  In the latter case, the word is parsed into list form before
it is run.  Thus, RUN READWORD or RUN READLIST will work.  The former is
slightly preferable because it allows for a continued line (with ~) that
includes a comment (with ;) on the first line.

RUN instructionlist

	command or operation.  Runs the Logo instructions in the input
	list; outputs if the list contains an expression that outputs.

WHILE tfexpression instructionlist			(library procedure)

	command.  Repeatedly evaluates the "instructionlist" as long as the
	evaluated "tfexpression" remains TRUE.  Evaluates the first input
	first, so the "instructionlist" may never be run at all.  The
	"tfexpression" must be an expressionlist whose value when evaluated
	is TRUE or FALSE.