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+# Introduction
+
+This document describes how we guard against accidental breaking
+changes in Subplot by running the current version against a curated
+set of subplot documents.
+
+# Subplot version 0.9.0
+
+## Produce HTML page
+
+~~~scenario
+given an installed subplot
+given a clone of https://gitlab.com/subplot/subplot.git in src at 5168420454b92205c13224a6801d3341d7f0c3d3
+when I docgen subplot.subplot to test.html, in src
+when I run, in src, subplot docgen subplot.subplot --merciful -o subplot.html -t python
+then file src/test.html exists
+~~~
+
+## Generate and run test program
+
+~~~scenario
+given an installed subplot
+given file run_test.sh
+given a clone of https://gitlab.com/subplot/subplot.git in src at 5168420454b92205c13224a6801d3341d7f0c3d3
+when I run, in src, subplot codegen subplot.subplot -o test-inner.py -t python
+when I run bash run_test.sh
+then command is successful
+~~~
+
+~~~{#run_test.sh .file .sh}
+#!/bin/bash
+
+set -euo pipefail
+
+N=100
+
+if python3 src/test-inner.py --log test-inner.log --env "PATH=$PATH" --env SUBPLOT_DIR=/
+then
+    exit=0
+else
+    exit="$?"
+fi
+
+if [ "$exit" != 0 ]
+then
+    # Failure. Show end of inner log file.
+
+    echo "last $N lines of test-inner.log:"
+    tail "-n$N" test-inner.log | sed 's/^/    /'
+fi
+
+exit "$exit"
+~~~