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+<!DOCTYPE html>
+
+<html lang="en">
+  <head>
+    <meta charset="utf-8">
+    <title>Curtailment - Packrat Docs</title>
+    <link href="default.css" rel="stylesheet">
+  </head>
+
+  <body>
+
+
+  <h2>Curtailment</h2>
+
+  <a href="index.html">Return to Contents</a>
+
+
+  <p><strong>CAUTION:</strong> This is implementation information and should not be
+  relied upon in any way when using the library.</p>
+
+  <p>A curtailment is information attached to a result denoting one or more
+  combinator/depth pairs.</p>
+
+  <p>Curtailment in general means that the result for a given combinator at a given
+  position is only valid when that combinator is next encountered at that position for
+  left recursion depth counts equal or greater than the curtailment.</p>
+
+  <p>If the left recursion count of a combinator at a particular position exceeds the
+  number of remaining input tokens plus one, the combinator is curtailed at that count
+  depth and fails.</p>
+
+  <p>When results are folded upwards, any curtailments are retained.</p>
+
+  <p>When two results are merged as in <em>Sequence</em> and <em>Choice</em>, if the
+  results share curtailments then the curtailment of greater depth is retained.</p>
+
+  <p>If a result has to be recalculated due to the previous result for a given
+  combinator at a given position not being valid due to being curtailed at a greater
+  left recursion depth, then the recalculated result has the previous result's
+  curtailments applied to it, except with the count depths replaced with the current
+  left recursion depths.</p>
+
+  <p>When a result is recalculated and the depth of a curtailment plus the number of
+  remaining tokens after the furthest finish of a result is less than the remaining
+  tokens at the start of a result, then the curtail can be deleted from the result,
+  since then it is no longer possible for curtailment to happen due to excessive
+  left recursion count depth. This is algebraically equivalent to checking if the
+  furthest finish of a result minus the left recursion depth is greater than the
+  start position of the result.</p>
+
+
+  </body>
+</html>
+