dest_type : hol_type -> string * hol_type list
# dest_type `:bool`;;
val it : string * hol_type list = ("bool", [])
# dest_type `:(bool)list`;;
val it : string * hol_type list = ("list", [`:bool`])
# dest_type `:num -> bool`;;
val it : string * hol_type list = ("fun", [`:num`; `:bool`])