Here's the compilation errors without it.
```
In file included from /home/tav/git/gtsam/CppUnitLite/TestHarness.h:23,
from /home/tav/git/gtsam/gtsam/discrete/tests/testDecisionTreeFactor.cpp:20:
/home/tav/git/gtsam/gtsam/discrete/tests/testDecisionTreeFactor.cpp: In member function ‘virtual void DecisionTreeFactorDivideTest::run(TestResult&)’:
/home/tav/git/gtsam/gtsam/discrete/tests/testDecisionTreeFactor.cpp:123:25: error: invalid initialization of reference of type ‘const gtsam::Vector&’ {aka ‘const Eigen::Matrix<double, -1, 1>&’} from expression of type ‘gtsam::DecisionTreeFactor’
123 | EXPECT(assert_inequal(pS, s));
| ^~
/home/tav/git/gtsam/CppUnitLite/Test.h:151:9: note: in definition of macro ‘EXPECT’
151 | { if (!(condition)) \
| ^~~~~~~~~
In file included from /home/tav/git/gtsam/gtsam/inference/DotWriter.h:22,
from /home/tav/git/gtsam/gtsam/inference/FactorGraph.h:25,
from /home/tav/git/gtsam/gtsam/inference/MetisIndex.h:21,
from /home/tav/git/gtsam/gtsam/inference/Ordering.h:25,
from /home/tav/git/gtsam/gtsam/discrete/DiscreteFactor.h:25,
from /home/tav/git/gtsam/gtsam/discrete/DecisionTreeFactor.h:22,
from /home/tav/git/gtsam/gtsam/discrete/tests/testDecisionTreeFactor.cpp:23:
/home/tav/git/gtsam/gtsam/base/Vector.h:163:48: note: in passing argument 1 of ‘bool gtsam::assert_inequal(const Vector&, const Vector&, double)’
163 | GTSAM_EXPORT bool assert_inequal(const Vector& vec1, const Vector& vec2, double tol=1e-9);
| ~~~~~~~~~~~~~~^~~~
/home/tav/git/gtsam/gtsam/discrete/tests/testDecisionTreeFactor.cpp:131:25: error: invalid initialization of reference of type ‘const gtsam::Vector&’ {aka ‘const Eigen::Matrix<double, -1, 1>&’} from expression of type ‘gtsam::KeySet’ {aka ‘gtsam::FastSet<long unsigned int>’}
131 | EXPECT(assert_inequal(KeySet(pS.keys()), keys));
| ^~~~~~~~~~~~~~~~~
/home/tav/git/gtsam/CppUnitLite/Test.h:151:9: note: in definition of macro ‘EXPECT’
151 | { if (!(condition)) \
| ^~~~~~~~~
/home/tav/git/gtsam/gtsam/base/Vector.h:163:48: note: in passing argument 1 of ‘bool gtsam::assert_inequal(const Vector&, const Vector&, double)’
163 | GTSAM_EXPORT bool assert_inequal(const Vector& vec1, const Vector& vec2, double tol=1e-9);
| ~~~~~~~~~~~~~~^~~~
```
```
In file included from /home/tav/git/gtsam/CppUnitLite/TestHarness.h:23,
from /home/tav/git/gtsam/gtsam/discrete/tests/testTableFactor.cpp:19:
/home/tav/git/gtsam/gtsam/discrete/tests/testTableFactor.cpp: In member function ‘virtual void TableFactorconstructorsTest::run(TestResult&)’:
/home/tav/git/gtsam/gtsam/discrete/tests/testTableFactor.cpp:147:25: error: invalid initialization of reference of type ‘const gtsam::Vector&’ {aka ‘const Eigen::Matrix<double, -1, 1>&’} from expression of type ‘gtsam::TableFactor’
147 | EXPECT(assert_inequal(f5_with_wrong_keys, f5, 1e-9));
| ^~~~~~~~~~~~~~~~~~
/home/tav/git/gtsam/CppUnitLite/Test.h:151:9: note: in definition of macro ‘EXPECT’
151 | { if (!(condition)) \
| ^~~~~~~~~
In file included from /home/tav/git/gtsam/gtsam/inference/DotWriter.h:22,
from /home/tav/git/gtsam/gtsam/inference/FactorGraph.h:25,
from /home/tav/git/gtsam/gtsam/inference/MetisIndex.h:21,
from /home/tav/git/gtsam/gtsam/inference/Ordering.h:25,
from /home/tav/git/gtsam/gtsam/discrete/DiscreteFactor.h:25,
from /home/tav/git/gtsam/gtsam/discrete/DecisionTreeFactor.h:22,
from /home/tav/git/gtsam/gtsam/discrete/DiscreteConditional.h:21,
from /home/tav/git/gtsam/gtsam/discrete/tests/testTableFactor.cpp:22:
/home/tav/git/gtsam/gtsam/base/Vector.h:163:48: note: in passing argument 1 of ‘bool gtsam::assert_inequal(const Vector&, const Vector&, double)’
163 | GTSAM_EXPORT bool assert_inequal(const Vector& vec1, const Vector& vec2, double tol=1e-9);
| ~~~~~~~~~~~~~~^~~~
```
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