/usr/include/ is in libixion-dev 0.13.0-2.
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..
/usr/include/libixion-0.13/
/usr/include/libixion-0.13/ixion/
/usr/include/libixion-0.13/ixion/address.hpp
/usr/include/libixion-0.13/ixion/cell.hpp
/usr/include/libixion-0.13/ixion/cell_listener_tracker.hpp
/usr/include/libixion-0.13/ixion/column_store_type.hpp
/usr/include/libixion-0.13/ixion/config.hpp
/usr/include/libixion-0.13/ixion/depth_first_search.hpp
/usr/include/libixion-0.13/ixion/env.hpp
/usr/include/libixion-0.13/ixion/exceptions.hpp
/usr/include/libixion-0.13/ixion/formula.hpp
/usr/include/libixion-0.13/ixion/formula_function_opcode.hpp
/usr/include/libixion-0.13/ixion/formula_name_resolver.hpp
/usr/include/libixion-0.13/ixion/formula_opcode.hpp
/usr/include/libixion-0.13/ixion/formula_result.hpp
/usr/include/libixion-0.13/ixion/formula_tokens.hpp
/usr/include/libixion-0.13/ixion/global.hpp
/usr/include/libixion-0.13/ixion/info.hpp
/usr/include/libixion-0.13/ixion/interface/
/usr/include/libixion-0.13/ixion/interface/formula_model_access.hpp
/usr/include/libixion-0.13/ixion/interface/session_handler.hpp
/usr/include/libixion-0.13/ixion/interface/table_handler.hpp
/usr/include/libixion-0.13/ixion/macros.hpp
/usr/include/libixion-0.13/ixion/matrix.hpp
/usr/include/libixion-0.13/ixion/mem_str_buf.hpp
/usr/include/libixion-0.13/ixion/model_context.hpp
/usr/include/libixion-0.13/ixion/table.hpp
/usr/include/libixion-0.13/ixion/types.hpp