Add missing ExtensionInterface.jl

Author: MilesCranmer

src/ExtensionInterface.jl

@@ -0,0 +1,18 @@
+module ExtensionInterfaceModule
+
+import ..EquationModule: Node, DEFAULT_NODE_TYPE
+import ..OperatorEnumModule: AbstractOperatorEnum
+import ..UtilsModule: isgood, isbad, @return_on_false
+
+function node_to_symbolic(args...; kws...)
+    return error(
+        "Please load the `SymbolicUtils` package to use `node_to_symbolic(::Node, ::AbstractOperatorEnum; kws...)`.",
+    )
+end
+function symbolic_to_node(args...; kws...)
+    return error(
+        "Please load the `SymbolicUtils` package to use `symbolic_to_node(::Symbolic, ::AbstractOperatorEnum, kws...)`.",
+    )
+end
+
+end