* src/variable.c: New file.
* simple.make (LIBMES_SOURCES): Add it.
* build-aux/configure-lib.sh (mes_SOURCES): Add it.
* build-aux/snarf.sh: Snarf it.
* kaem.run: Compile it.
* include/mes/builtins.h (make_variable): New function.
(variable_p): New function.
(variable_ref): New function.
(variable_set_x): New function.
(variable_printer): New function.
* src/builtins.c (mes_builtins): Register them.
* include/mes/mes.h (make_variable_type): New function.
(scm_variable_type): New variable.
* src/module.c (make_initial_module): Initialize it.
* src/gc.c (gc_flip, gc_): Keep track of it.
* include/mes/symbols.h (cell_symbol_variable): New variable.
(SYMBOL_MAX): Adjust accordingly.
* src/symbol.c (init_symbols): Initialize 'cell_symbol_variable'.
* mes/module/mes/scm.mes (make-undefined-variable): New procedure.
(variable-bound?): New procedure.
* tests/variable.test: New file.
* build-aux/check-mes.sh (TESTS): Add it.
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