The if-else cases in lsqr.py were not exhaustive. They become exhaustive by changing ab > aa to ab >= aa.
set (diagram-viewerSources
diagram.cpp
diagram-viewer-main.cpp)
set (diagram-viewerHeaders
diagram.h)
qt4_wrap_cpp (diagram-viewerMocSources ${diagram-viewerHeaders})
set (libraries ${libraries}
${Boost_SERIALIZATION_LIBRARY}
${Boost_PROGRAM_OPTIONS_LIBRARY}
${QT_LIBRARIES})
add_executable (diagram-viewer ${diagram-viewerSources}
${diagram-viewerMocSources})
target_link_libraries (diagram-viewer ${libraries})