Suppose text data has 3 constructs: a, b, and c. * a must occur 1 time * b can occur 0 or 1 time * c can occur any number of times, 0 or more These 3 constructs can appear in any order. So these are valid inputs: abc a bcccca But these are not: ccbcc abbc abcabc Can data like this be modeled with DFDL? The unordered sequence can be modeled with a data array, like this: Array (0 to unbounded) Sequence a (0 to 1) b (0 to 1) c (0 to 1) /Sequence /Array But I don't know how to constrain the total number of occurrences. Appreciate any ideas!