fix: improve plain-text (#57)

* fix #55: improve `plain-text`

* fix: store local variables for performance & readability

contrib/typst/book/lib.typ

@@ -135,16 +135,26 @@
 
 // Collect text content of element recursively into a single string
 // https://discord.com/channels/1054443721975922748/1088371919725793360/1138586827708702810
+// https://github.com/Myriad-Dreamin/typst-book/issues/55
+#let _styled = smallcaps("").func();
+#let _equation = $1$.func();
+#let _sequence = [].func();
 #let plain-text(it) = {
   if type(it) == str {
-    it
+    return it
   } else if it == [ ] {
-    " "
-  } else if it.func() == text or it.func() == raw {
+    return " "
+  }
+  let f = it.func()
+  if f == _styled {
+    plain-text(it.child)
+  } else if f == _equation {
+    plain-text(it.body)
+  } else if f == text or f == raw {
     it.text
-  } else if it.func() == smartquote {
+  } else if f == smartquote {
     if it.double { "\"" } else { "'" }
-  } else if it.func() == [].func() {
+  } else if f == _sequence {
     it.children.map(plain-text).filter(t => type(t) == str).join()
   } else {
     none