Control Structures

Control words are used in
Explicit Definition (:)
and are punctuation that determine the
sequence of execution.
Matching control words and the enclosed sentences
make up a control structure. The following control words and
structures are available:

assert. T
for.     T do. B end.
for_xyz. T do. B end.
if. T do. B end.
if. T do. B else. B1 end.
if.     T  do. B
elseif. T1 do. B1
elseif. T2 do. B2
select. T
 case.  T0 do. B0
 fcase. T1 do. B1
 case.  T2 do. B2
try. B catch. B1 catchd. B2 catcht. B3 end.
while.  T do. B end.
whilst. T do. B end.

Words beginning with B or T
denote blocks, comprising zero or more simple sentences
and control structures. The last sentence executed in
a T block is tested for a non-zero value in its
leading atom, and determines the block to be executed next.
(An empty T block result or an omitted T
block tests true.)
The final result is the result of the last sentence
executed that was not in a T block, and
if there is no such last executed sentence,
the final result is i.0 0 .

These control words and control structures are further
detailed in the immediately following pages.