change name of attr

font-features-remove is consistent with set-remove

quad2/constants.rkt

@@ -1,10 +1,12 @@
 #lang racket/base
 (require racket/list
+         racket/set
          "struct.rkt")
 (provide (all-defined-out))
 
 (define default-font-family "text")
 (define default-font-size 12)
+(define default-features (seteq))
 
 (define-syntax-rule (define-attr-list LIST-NAME
                       [ATTR-NAME ATTR-EXPR] ...)
@@ -25,6 +27,6 @@
   [:font-bold (make-attr-boolean-key 'font-bold #true #false)]
   [:font-italic (make-attr-boolean-key 'font-italic #true #false)]
   [:font-size (make-attr-dimension-string-key 'font-size #true default-font-size)]
-  [:font-features (make-attr-set-key 'font-features)]
-  [:font-features-add (make-attr-set-key 'font-features-add)]
-  [:font-features-subtract (make-attr-set-key 'font-features-subtract)])
+  [:font-features (make-attr-set-key 'font-features default-features)]
+  [:font-features-add (make-attr-set-key 'font-features-add default-features)]
+  [:font-features-remove (make-attr-set-key 'font-features-remove default-features)])