423 moving the focus there are three ways to change

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Unformatted text preview: to its right indicates a menu where constants can be added or removed from this box. Rule menu icon. This indicates a popup menu where you can choose a rule to justify a step of the proof. 4.2.1 Step numbers Normally, Fitch does not display step numbers, but simply indicates the steps with bullets. Choosing Show Step Numbers from the Proof menu will replace the bullets with numbers. When you display step numbers, the support steps are indicated by number next to the rule name, exactly as they are shown in the text. 4.2.2 The current focus As you work on a proof, there is always one step that is “in focus.” This step is indicated by a red triangle on the far left of the proof, called the Focus Slider. The focused step is the step affected when you perform any of the editing functions. It is also the step that is checked if you click on the Check Step button or on the status line at the very bottom of the proof window. 4.2.3 Moving the focus There are three ways to change the focus from one step to another: You can drag the focus slider up and down, you can click in the Using Fitch / 35 focus slider area next to the step you want to focus on, or you can use the arrow keys on the keyboard to move the focus up and down. Clicking on a step other than the currently focused step won’t move the focus, unless you click in the focus slider area to the left. This is because clicking on steps is the way we enter supports for the currently focused step. (See page 37.) 4.2.4 Adding new steps To add a new step to a proof, choose Add Step After or Add Step Before from the Proof menu. These commands will give you a new proof step immediately after or before the step you were focused on, unless you were focused on a premise, in which case the new step will be the first step following the premises. To add a step at the end of the proof, focus on the last step of the proof and choose Add Step After. If you are working backwards in a proof, you will often want to add a step immediately before the step in focus, using Add Step Before. Steps can be inserted in the middle of a proof in the obvious way, by first moving the focus and then choosing the appropriate add step command. New steps added within a subproof will appear in the same subproof. Starting and ending subproofs require different commands. These are described on page 39. Adding premises to a proof is discussed on page 43. Normally, though, you will not be adding premises to your proofs, since the exercise files contain the premises already. 4.2.5 Entering sentences into steps When you add a new step to a proof, the step icon (a small circle) will appear. At this point you can enter a sentence. To enter a sentence, either use the tool bar or type directly from the keyboard. To type the logical symbols from the keyboard, refer to the list of keyboard equivalents found in Table 3.1 on page 23. These are the same keyboard equivalents used in Tarski’s World and Boole. In general, entering sentences...
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