Thus with step numbers displayed your proofs will

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Unformatted text preview: g a step’s rule To change the rule of an existing step, you must first move the focus to that step. Then specify the new rule using either the popup menu or the keystroke equivalents. 4.2.9 Specifying a step’s supports Most rules require that you cite other steps as justification or “support.” To specify the supports for a step, focus on that step and 38 / LPL Software Manual click on the steps to be cited. The steps you click on will highlight. If your support is a subproof, clicking anywhere in the subproof will highlight the whole subproof. If you click on a step or subproof that has already been cited, it will be uncited. To see a step’s supports, just focus on the step in question. The supporting steps will then become highlighted. To change a step’s supports, focus on the step and click on the steps you wish to add or delete from the step’s supports. If you are displaying step numbers in a proof, then the support steps are not indicated with highlighting, but rather by step numbers appearing to the right of the rule name. Thus with step numbers displayed, your proofs will look like the proofs in the text. 4.2.10 Checking steps and verifying proofs To check whether a step is correct, focus on the step and either press the Check Step button on the toolbar or click on the status line at the bottom of the window. (On the Macintosh, you can also check a step by hitting the Enter key on the numeric keypad.) You can check all of the steps in your proof, plus the goals, by clicking Verify Proof on the toolbar or by choosing Verify Proof on the Proof menu. After you check a step, one of four symbols will appear to the left of the rule name. Check mark. A check mark means that the step is logically correct. X. An X means that the step is logically incorrect. Asterisk. An asterisk means that the sentence at that step is not syntactically well-formed. Question mark. A question mark will appear if a Con rule is unable to determine the validity of your step. If you don’t get a check mark for one of your steps, focus on that step and look at the message in the status line. With luck, it will provide you with some helpful information about why your step did not check out. Using Fitch / 39 4.2.11 Rule Defaults Many of rules have defaults that can save you considerable time when constructing a proof. For example, if you choose the rule → Elim, cite two sentences of the form P → Q and P, and then check the step, Fitch will automatically fill in the step with the sentence Q. To get Fitch to provide a default for a step, the sentence must be blank, that is, there must not be any text already in that step. If the sentence is blank when the step is checked, Fitch will try to provide a default sentence for that step. The defaults for the rules are described in detail in the textbook. The Taut Con, FO Con, and Ana Con procedures do not have defaults. 4.2.12 Add Support Steps Many of the rules allow you to use the Add Support Steps command to automatically insert the appropriate support steps needed to derive a particular fo...
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