Attributes
v_true n is v n "", the boolean attribute n set to true (the attribute must be omitted to be false).
add_if c att atts is att :: atts if c is true and atts otherwise.
add_some n o atts is (v n value) :: atts if o is Some value and atts otherwise.
Predefined attribute constructors
type 'a cons = 'a -> tThe type for attribute constructors with value of type 'a.
val placeholder : string cons