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Consider the definition of `cut_direct` below (part of `propRule.key`).
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```
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\schemaVariables {
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\formula cutFormula;
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}
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\rules {
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cut_direct {
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\find(cutFormula)
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\sameUpdateLevel
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"CUT: #cutFormula TRUE" [main]:
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\replacewith(true) \add(cutFormula ==>);
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"CUT: #cutFormula FALSE":
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\replacewith(false) \add( ==> cutFormula)
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\heuristics(cut_direct)
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};
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}
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```
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First, we need to define the variables we want to use in the taclet.
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This is the `\schemaVariables` block at the start of the `.key` file.
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We only require a single `\formula` type schema variable called `cutFormula`.
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Then, we define the actual taclets in this file by creating a `\rules` block.
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For the purposes of this example, we limit ourselves to the cut_direct taclet.
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`propRule.key` contains many more taclets.
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The `\rules` block contains the taclet definitions, each of which begins with the name of the taclet
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("cut_direct"), creating a new block defined by curly brackets and a semicolon at the end.
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Since the `\find(...)` part of the taclet definition does not contain a `==>`, the `cut_direct` taclet finds (matches) a sub-term anywhere in a sequent formula.
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The `\sameUpdateLevel` essentially ensures it doesn't match under an update application.
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The taclet creates two new branches, which are defined directly afterwards.
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The first branch is labeled "CUT: #cutFormula TRUE".
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In this branch, the found sub-term is replaced with true (`\replacewith(true)`), and the found sub-term is added as a new sequent formula to the antecedent: `\add(cutFormula ==>)`.
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A particular branch of the taclet can be tagged by enclosing the tag in brackets.
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This tag must be written after the branch label.
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The first branch in this example is tagged with "main".
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This particular value causes the branch to be visually continued on the parent branch if [the linearized Proof Tree mode](../../user/ProofTreeLinearMode/) is active.
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The second branch of the taclet is labeled "CUT: #cutFormula FALSE".
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In this branch, the found sub-term is replaced with false (`\replacewith(false)`), and the found sub-term is added as a new sequent formula to the succedent: `\add( ==> cutFormula)`.
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Finally, the taclet is added to the `cut_direct` heuristics group.
In the proof tree settings, you can enable the "Linearize Proof Tree" option.
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## Effect when enabled
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For symbolic execution steps, the "Normal Execution" branch will (visually) continue on the parent branch. See the screenshot below for a quick example.
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The exceptional case branches (e.g. Null Reference, Index Out of Bounds) are therefore placed above the steps of the Normal Execution branch.
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Additionally, the TRUE branch of cut_direct applications will visually continue on the parent branch.
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This mechanism may be extended further by other taclets tagging their "main" created branch.
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