Arlington County has an extensive network of bicycle infrastructure, including 50 miles of shared-use off-street trails. These multi-use trails are generally 10 feet wide with a solid yellow line striped down the middle to separate users.
Constructive mathematics. Idea. Broadly speaking, constructive mathematics is mathematics done without the principle of excluded middle (or other principles, such as the full axiom of choice, that imply it).
Formalist definitions identify mathematics with its symbols and the rules for operating on them. Haskell Curry defined mathematics simply as the science of formal systems. A formal system is a set of symbols, or tokens, and some rules telling how the tokens may be combined into formulas.