feat: Display math equation numbers now use a resilient style (eqno)

classes/resilient/book.lua

@@ -92,6 +92,37 @@ function class:_init (options)
   self:registerCommand("urlstyle", function (_, content)
     SILE.call("style:apply", { name = "url" }, content)
   end)
+
+  -- Override the standard math:numberingstyle hook to rely on styles,
+  -- and also to subscribe for cross-references.
+  -- Package "math" is loaded by the markdown package.
+  self:registerCommand("math:numberingstyle", function (options, _)
+    local text
+    if options.counter then
+      local fnSty = self:resolveStyle("eqno")
+      local display = fnSty.numbering and fnSty.numbering.display or "arabic"
+      -- Enforce the display style for the counter
+      -- (which might be the default 'equation' counter, or a custom one)
+      SILE.call("set-counter", { id = options.counter, display = display })
+      text = self.packages.counters:formatCounter(SILE.scratch.counters[options.counter])
+    elseif options.number then
+      text = options.number
+    else
+      SU.error("No counter or number provided for math numbering")
+    end
+    -- Cross-ref support (from markdown, we can get an id, in most of our packages,
+    -- we can get a marker, play fair with all)
+    local mark = options.id or options.marker
+    if mark then
+      local labelRefs = self.packages.labelrefs
+      labelRefs:pushLabelRef(text)
+      SILE.call("style:apply:number", { name = "eqno", text = text })
+      SILE.call("label", { marker = mark })
+      labelRefs:popLabelRef()
+    else
+      SILE.call("style:apply:number", { name = "eqno", text = text })
+    end
+  end)
 end
 
 function class:declareOptions ()
@@ -382,6 +413,15 @@ function class:registerStyles ()
   self:registerStyle("url", { inherit = "code"}, {
   })
 
+  -- display math equation numberstyle
+  self:registerStyle("eqno", {}, {
+    numbering = {
+      before = { text = "(" },
+      display = "arabic",
+      after = { text = ")" }
+    }
+  })
+
   -- Special non-standard style for dropcaps (for commands initial-joined and initial-unjoined)
   self:registerStyle("dropcap", {}, {
     font = { family = "Zallman Caps" },

examples/manual-styling/advanced/other.md

@@ -9,5 +9,6 @@ Readers experienced with SILE’s standard packages also have to be aware that t
 For instance, the (standard) **url** package provides a `urlstyle` hook command. While you could override it---that is, redefine that command---, please note that this is what the resilient classes already do, so as to delegate the decision to a style conveniently called `urlstyle` too.[^other-sile-hooks]
 
 [^other-sile-hooks]: The same principle actually applies to page numbers, relying on the standard **folio** package. The latter provides a `foliostyle` hook, which the resilient styling system overrides with its own clever logic.
+Likewise, the `math:numberstyle` hook from the **math** package is also redefined, to rely on the `eqno` style for display math equation numbers.
 
 In other terms, even for standard SILE packages, we may already have provided a style-aware version of their original customization hooks.