+(deftest examples
+ (is (= 31 (find-lowest-exchange-rate (make-graph ["10 ORE => 10 A"
+ "1 ORE => 1 B"
+ "7 A, 1 B => 1 C"
+ "7 A, 1 C => 1 D"
+ "7 A, 1 D => 1 E"
+ "7 A, 1 E => 1 FUEL"])
+ "FUEL", "ORE")))
+ (is (= 165 (find-lowest-exchange-rate (make-graph ["9 ORE => 2 A"
+ "8 ORE => 3 B"
+ "7 ORE => 5 C"
+ "3 A, 4 B => 1 AB"
+ "5 B, 7 C => 1 BC"
+ "4 C, 1 A => 1 CA"
+ "2 AB, 3 BC, 4 CA => 1 FUEL"])
+ "FUEL", "ORE")))
+ (is (= 13312 (find-lowest-exchange-rate (make-graph ["157 ORE => 5 NZVS"
+ "165 ORE => 6 DCFZ"
+ "44 XJWVT, 5 KHKGT, 1 QDVJ, 29 NZVS, 9 GPVTF, 48 HKGWZ => 1 FUEL"
+ "12 HKGWZ, 1 GPVTF, 8 PSHF => 9 QDVJ"
+ "179 ORE => 7 PSHF"
+ "177 ORE => 5 HKGWZ"
+ "7 DCFZ, 7 PSHF => 2 XJWVT"
+ "165 ORE => 2 GPVTF"
+ "3 DCFZ, 7 NZVS, 5 HKGWZ, 10 PSHF => 8 KHKGT"])
+ "FUEL", "ORE")))
+ (is (= 180697 (find-lowest-exchange-rate (make-graph ["2 VPVL, 7 FWMGM, 2 CXFTF, 11 MNCFX => 1 STKFG"
+ "17 NVRVD, 3 JNWZP => 8 VPVL"
+ "53 STKFG, 6 MNCFX, 46 VJHF, 81 HVMC, 68 CXFTF, 25 GNMV => 1 FUEL"
+ "22 VJHF, 37 MNCFX => 5 FWMGM"
+ "139 ORE => 4 NVRVD"
+ "144 ORE => 7 JNWZP"
+ "5 MNCFX, 7 RFSQX, 2 FWMGM, 2 VPVL, 19 CXFTF => 3 HVMC"
+ "5 VJHF, 7 MNCFX, 9 VPVL, 37 CXFTF => 6 GNMV"
+ "145 ORE => 6 MNCFX"
+ "1 NVRVD => 8 CXFTF"
+ "1 VJHF, 6 MNCFX => 4 RFSQX"
+ "176 ORE => 6 VJHF"])
+ "FUEL", "ORE")))
+ (is (= 2210736 (find-lowest-exchange-rate (make-graph ["171 ORE => 8 CNZTR"
+ "7 ZLQW, 3 BMBT, 9 XCVML, 26 XMNCP, 1 WPTQ, 2 MZWV, 1 RJRHP => 4 PLWSL"
+ "114 ORE => 4 BHXH"
+ "14 VRPVC => 6 BMBT"
+ "6 BHXH, 18 KTJDG, 12 WPTQ, 7 PLWSL, 31 FHTLT, 37 ZDVW => 1 FUEL"
+ "6 WPTQ, 2 BMBT, 8 ZLQW, 18 KTJDG, 1 XMNCP, 6 MZWV, 1 RJRHP => 6 FHTLT"
+ "15 XDBXC, 2 LTCX, 1 VRPVC => 6 ZLQW"
+ "13 WPTQ, 10 LTCX, 3 RJRHP, 14 XMNCP, 2 MZWV, 1 ZLQW => 1 ZDVW"
+ "5 BMBT => 4 WPTQ"
+ "189 ORE => 9 KTJDG"
+ "1 MZWV, 17 XDBXC, 3 XCVML => 2 XMNCP"
+ "12 VRPVC, 27 CNZTR => 2 XDBXC"
+ "15 KTJDG, 12 BHXH => 5 XCVML"
+ "3 BHXH, 2 VRPVC => 7 MZWV"
+ "121 ORE => 7 VRPVC"
+ "7 XCVML => 6 RJRHP"
+ "5 BHXH, 4 VRPVC => 5 LTCX"])
+ "FUEL", "ORE"))))