User interface > Debugging and executing > Execute a look-ahead step-by-step
Productions can be executed on a trial basis. In dependence of the success the same production is executed or another branch is chosen. Such a look-ahead also can be tested step-by-step.
This is possible in arbitrary staggering. I.e. the success of a look-ahead can depend on further look-ahead's which are carried out within the first one.
Level of look-ahead
The level of the look-ahead is shown on a little field within the tool bar. An empty field or a zero means that the parser isn't within a look-ahead but in the main stream.
Into the look-ahead (Ctrl + F7)
If the parser is at the beginning of an IF or a WHILE structure like shown below,
you can step into the corresponding look-ahead by this button. At other positions the button works just like the button for a single step within a look-ahead level. If the expected token doesn't belong to the first set of the look-ahead-production, there is no change of the level too.
Out of the look-ahead (Ctrl + F8)
You can leave a look-ahead by this button. All remaining steps within the current level are executed at once and the parser stops at the next higher level.
Remark: you can use the other buttons and functions of the debugger the same way within a level of a look-ahead like at the level of the main parser.