Goal ::= <. arb n .>
         <. a = 0 .> { "a" <. a += 1 .> } <. a = n .>
         <. b = 0 .> { "b" <. b += 1 .> } <. b = n .>
         <. c = 0 .> { "c" <. c += 1 .> } <. c = n .>
         ;