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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

    break.

    continue.

    for.     T do. B end.
    for_xyz. T do. B end.

    goto_name.
    label_name.

    if. T do. B end.

    if. T do. B else. B1 end.

    if.     T  do. B
    elseif. T1 do. B1
    elseif. T2 do. B2
    end.

    return.

    select. T
     case.  T0 do. B0
     fcase. T1 do. B1
     case.  T2 do. B2
    end.

    throw.

    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.

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