The type of guest-level values

The type of guest-level types

default_value t should return a default value for type t

vcd_type v should return the VCD type corresponding to value v. For example, if v denotes an boolean value, vcd_type v should return Rfsm.Vcd_types.TyBool. This function should raise Unsupported_vcd in case of failure (if there's no corresponding VCD type)

vcd_value v should return the VCD encoding of value v. For example, if v denotes the boolean value b, vcd_value v should return Rfsm.Vcd_types.Val_bool b. This function should raise Unsupported_vcd in case of failure (if there's no corresponding VCD value)

flatten base:b v should decompose a structured value v into a list of qualified scalar values for VCD dumping. For example, if v is a record {x=1;y=2}, then flatten ~base:"a" v should be [("a.x",1);("a.y",2)]. If v is a scalar value, then flatten ~base:"a" v is just ["a",v]

pp fmt v prints value v on formatter fmt.