Fix typo in definition of tetrahedron (#173)

* fix cell typo

* Add test

test/test_cell.py

@@ -0,0 +1,61 @@
+import ufl
+import pytest
+
+
+def test_interval():
+    cell = ufl.interval
+    assert cell.num_vertices() == 2
+    assert cell.num_edges() == 1
+    assert cell.num_faces() == 0
+
+
+def test_triangle():
+    cell = ufl.triangle
+    assert cell.num_vertices() == 3
+    assert cell.num_edges() == 3
+    assert cell.num_faces() == 1
+
+
+def test_quadrilateral():
+    cell = ufl.quadrilateral
+    assert cell.num_vertices() == 4
+    assert cell.num_edges() == 4
+    assert cell.num_faces() == 1
+
+
+def test_tetrahedron():
+    cell = ufl.tetrahedron
+    assert cell.num_vertices() == 4
+    assert cell.num_edges() == 6
+    assert cell.num_faces() == 4
+
+
+def test_hexahedron():
+    cell = ufl.hexahedron
+    assert cell.num_vertices() == 8
+    assert cell.num_edges() == 12
+    assert cell.num_faces() == 6
+
+
+
+@pytest.mark.parametrize("cell", [ufl.interval])
+def test_cells_1d(cell):
+    assert cell.num_facets() == cell.num_vertices()
+    assert cell.num_ridges() == 0
+    assert cell.num_peaks() == 0
+
+
+@pytest.mark.parametrize("cell", [ufl.triangle, ufl.quadrilateral])
+def test_cells_2d(cell):
+    assert cell.num_facets() == cell.num_edges()
+    assert cell.num_ridges() == cell.num_vertices()
+    assert cell.num_peaks() == 0
+
+
+@pytest.mark.parametrize("cell", [ufl.tetrahedron, ufl.hexahedron, ufl.prism, ufl.pyramid])
+def test_cells_2d(cell):
+    assert cell.num_facets() == cell.num_faces()
+    assert cell.num_ridges() == cell.num_edges()
+    assert cell.num_peaks() == cell.num_vertices()
+
+

ufl/cell.py

@@ -194,7 +194,7 @@ def peak_types(self) -> typing.Tuple[AbstractCell, ...]:
     "interval": [tuple("vertex" for i in range(2)), ("interval", )],
     "triangle": [tuple("vertex" for i in range(3)), tuple("interval" for i in range(3)), ("triangle", )],
     "quadrilateral": [tuple("vertex" for i in range(4)), tuple("interval" for i in range(4)), ("quadrilateral", )],
-    "tetrahedron": [tuple("vertex" for i in range(4)), tuple("interval" for i in range(4)),
+    "tetrahedron": [tuple("vertex" for i in range(4)), tuple("interval" for i in range(6)),
                     tuple("triangle" for i in range(4)), ("tetrahedron", )],
     "hexahedron": [tuple("vertex" for i in range(8)), tuple("interval" for i in range(12)),
                    tuple("quadrilateral" for i in range(6)), ("hexahedron", )],
@@ -251,20 +251,26 @@ def has_simplex_facets(self) -> bool:
 
     def num_sub_entities(self, dim: int) -> int:
         """Get the number of sub-entities of the given dimension."""
+        if dim < 0:
+            return 0
         try:
             return self._num_cell_entities[dim]
         except IndexError:
             return 0
 
     def sub_entities(self, dim: int) -> typing.Tuple[AbstractCell, ...]:
         """Get the sub-entities of the given dimension."""
+        if dim < 0:
+            return ()
         try:
             return self._sub_entities[dim]
         except IndexError:
             return ()
 
     def sub_entity_types(self, dim: int) -> typing.Tuple[AbstractCell, ...]:
         """Get the unique sub-entity types of the given dimension."""
+        if dim < 0:
+            return ()
         try:
             return self._sub_entity_types[dim]
         except IndexError: