diff --git a/tests/test_bursts_fortran.py b/tests/test_bursts_fortran.py
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+"""Burst segmentation against the rule transcribed from the Fortran.
+
+``dcio.analysis.bursts`` is checked here against an independent implementation
+of the same definition, transcribed from ``hjclik.for`` in
+``DCPROGS/DCFORTRAN`` (``Fort90/HJCFIT/``) -- the likelihood that produced
+every published HJCFIT result, Colquhoun, Hatton & Hawkes (2003) among them.
+
+``test_bursts.py`` fixes the conventions by stating them; this fixes them
+against a separate reading of the original, which is a different kind of
+evidence. The three implementations in the DCPROGS stack disagreed on exactly
+these points, and agreeing with each other would not have settled which was
+right.
+
+The Fortran does not pre-split a record. ``hjclik.for`` line 1009 walks it and
+ends the group when a shut time exceeds ``tcrit``::
+
+    if(tint(in,jset).gt.tcrit(jset)) goto 92   !end present group with prev opening
+
+Three things follow, and each is a separate test below:
+
+* the comparison is **strictly greater**, so a gap exactly equal to ``tcrit``
+  is within-burst;
+* the group ends *with the previous opening*, so the separating gap belongs to
+  no group and every group starts and ends on an opening;
+* the test is **not guarded by** ``burst(jset)``. A ``tcrit`` of zero therefore
+  ends the group at every shut time. HJCFIT means "do not divide" by setting
+  ``tcrit`` enormous -- 3.1536e10 ms, one year (``Hjcfit1-09122003.for`` line
+  1568) -- not by setting it to zero. That trap cost real time to find and is
+  worth a test of its own.
+"""
+
+import numpy as np
+import numpy.testing as npt
+import pytest
+
+from dcio.analysis.bursts import extract_burst_intervals
+
+TCRIT = 3.5e-3
+
+
+def fortran_bursts(tints, ampls, tcrit):
+    """``hjclik.for`` line 1009, transcribed.
+
+    A shut time greater than ``tcrit`` ends the group with the previous
+    opening. Returns a list of interval arrays, each starting and ending on an
+    opening.
+    """
+    t = np.asarray(tints, float)
+    a = np.asarray(ampls, float)
+    groups, cur = [], []
+    for ti, ai in zip(t, a):
+        if ai == 0.0 and ti > tcrit:            # strictly greater
+            if cur:
+                groups.append(cur)
+            cur = []
+        else:
+            cur.append((ti, ai))
+    if cur:
+        groups.append(cur)
+
+    out = []
+    for g in groups:                            # "with prev opening"
+        while g and g[0][1] == 0.0:
+            g = g[1:]
+        while g and g[-1][1] == 0.0:
+            g = g[:-1]
+        if g:
+            out.append(np.array([x[0] for x in g]))
+    return out
+
+
+def alternating(n, seed, tcrit=TCRIT):
+    """A record that alternates open and shut, with gaps either side of tcrit."""
+    rng = np.random.default_rng(seed)
+    t = np.empty(n)
+    a = np.zeros(n)
+    for i in range(n):
+        if i % 2 == 0:                          # opening
+            t[i] = rng.exponential(3e-4)
+            a[i] = 5.0
+        else:                                   # shut: a mixture, so that
+            t[i] = (rng.exponential(2e-4) if rng.random() < 0.75
+                    else rng.exponential(2e-2))  # some gaps exceed tcrit
+    return t, a
+
+
+@pytest.mark.parametrize("seed", [0, 1, 2, 3, 4])
+def test_agrees_with_the_fortran_rule(seed):
+    t, a = alternating(4000, seed)
+    got = extract_burst_intervals(t, a, TCRIT)
+    want = fortran_bursts(t, a, TCRIT)
+
+    assert len(got) == len(want)
+    for g, w in zip(got, want):
+        npt.assert_allclose(np.asarray(g, float), w, rtol=0, atol=0)
+    assert len(want) > 10, "the record should actually contain bursts"
+
+
+@pytest.mark.parametrize("tcrit", [1e-4, 5e-4, 3.5e-3, 2e-2])
+def test_agrees_at_every_tcrit(tcrit):
+    t, a = alternating(2000, seed=7)
+    got = extract_burst_intervals(t, a, tcrit)
+    want = fortran_bursts(t, a, tcrit)
+    assert len(got) == len(want)
+    for g, w in zip(got, want):
+        npt.assert_allclose(np.asarray(g, float), w, rtol=0, atol=0)
+
+
+def test_the_comparison_is_strictly_greater():
+    """A gap exactly equal to tcrit is within-burst, as line 1009 has it."""
+    t = np.array([1e-4, TCRIT, 1e-4, TCRIT * 1.000001, 1e-4])
+    a = np.array([5.0, 0.0, 5.0, 0.0, 5.0])
+
+    got = extract_burst_intervals(t, a, TCRIT)
+    want = fortran_bursts(t, a, TCRIT)
+    assert len(got) == len(want) == 2, "only the longer gap should split"
+    npt.assert_allclose(np.asarray(got[0], float), want[0])
+
+
+def test_every_burst_starts_and_ends_on_an_opening():
+    """"end present group with prev opening" -- the separating gap is dropped."""
+    t, a = alternating(2000, seed=11)
+    for g in extract_burst_intervals(t, a, TCRIT):
+        g = np.asarray(g, float)
+        assert g.size % 2 == 1, "odd length means opening at both ends"
+
+
+def test_tcrit_of_zero_splits_at_every_shut_time():
+    """The trap: line 1009 is not guarded by burst(jset).
+
+    HJCFIT means "do not divide" by setting tcrit to a year, not to zero. A
+    caller who passes zero expecting "no division" gets one group per opening,
+    silently, and this records that both implementations behave that way.
+    """
+    t, a = alternating(200, seed=13)
+    got = extract_burst_intervals(t, a, 0.0)
+    want = fortran_bursts(t, a, 0.0)
+    assert len(got) == len(want)
+    assert all(np.asarray(g, float).size == 1 for g in got)
+    assert len(got) == int((a != 0).sum())
+
+
+def test_an_enormous_tcrit_is_how_you_ask_for_one_group():
+    """3.1536e10 ms, one year: Hjcfit1-09122003.for line 1568."""
+    t, a = alternating(200, seed=17)
+    one_year = 3.1536e10 * 1e-3                  # ms -> s
+    got = extract_burst_intervals(t, a, one_year)
+    want = fortran_bursts(t, a, one_year)
+    assert len(got) == len(want) == 1
+    npt.assert_allclose(np.asarray(got[0], float), want[0])