Added all source files

operand-var.go

@@ -0,0 +1,32 @@
+// operand-var.go
+package expr
+
+import "fmt"
+
+// -------- variable term
+func newVarTerm(tk *Token) *term {
+	return &term{
+		tk:       *tk,
+		class:    classVar,
+		kind:     kindUnknown,
+		parent:   nil,
+		children: nil,
+		position: posLeaf,
+		priority: priValue,
+		evalFunc: evalVar,
+	}
+}
+
+// -------- eval func
+func evalVar(ctx exprContext, self *term) (v any, err error) {
+	var exists bool
+	if v, exists = ctx.GetValue(self.tk.source); !exists {
+		err = fmt.Errorf("undefined variable %q", self.tk.source)
+	}
+	return
+}
+
+// init
+func init() {
+	registerTermConstructor(SymIdentifier, newVarTerm)
+}