Implement adding expression and var decls

src/support/page.ts

@@ -16,6 +16,11 @@ export class MathPage {
         public readonly id: string,
     ) {}
 
+    /** Get all defined statements. */
+    getStatements(): MathStatement[] {
+        return Object.values(this.statements)
+    }
+
     /** Get a statement by ID if it exists. */
     getStatement(id: StatementID): Maybe<MathStatement> {
         return this.statements[id]

src/support/parse.ts

@@ -324,4 +324,18 @@ export class MathStatement {
     uses(): math.SymbolNode[] {
         return (new RValSymbolWalk()).walk(this.parse())
     }
+
+    /** Returns true if the definition is correctly non-recursive. */
+    isNotRecursive(): boolean {
+        const uses = this.uses()
+        const defines = this.defines()
+
+        for ( const define of defines ) {
+            if ( uses.some(x => x.name === define.name) ) {
+                return false
+            }
+        }
+
+        return true
+    }
 }