Within standard ASDL, the sequences already have an ASDL type.  Here's a snippet that shows both builtin types and derived types:

start = Program(stmt* procs, expr* calls)

stmt = Procedure(identifier name, identifier* params, bool is_test, stmt body)
     | Block(int blocknum, stmt* stmts)
     | Assign(lvalue target, expr value)
     | If(expr cond, stmt action)
     | Loop(int times, bool fixed, body stmt)
     | AbortLoop(int blocknum)
     | QuitBlock(int blocknum)

expr = BinOp(expr value1, str op, expr value2)
     | Number(int x)
     | Bool(bool x)
     | Id(identifier name)
     | Call(identifier name, expr* args)
     | lvalue

lvalue = Output(bool is_bool)
       | Cell(int i)