diff --git a/t_helpers_test.go b/t_helpers_test.go
index 4f8deae..53bd1ee 100644
--- a/t_helpers_test.go
+++ b/t_helpers_test.go
@@ -5,51 +5,55 @@
 package expr
 
 import (
+	"fmt"
+
 	"testing"
+
+	"git.portale-stac.it/go-pkg/expr/kern"
 )
 
-// TODO The new function param model does not allow this kind of test
-// ------------------------------------------------------------------
-// func subtract(ctx ExprContext, name string, args map[string]any) (result any, err error) {
-// 	if len(args) != 2 {
-// 		err = fmt.Errorf("%s(): requires exactly two arguments", name)
-// 		return
-// 	}
-// 	x, xok := args[0].(int64)
-// 	y, yok := args[1].(int64)
-// 	if xok && yok {
-// 		result = x - y
-// 	} else {
-// 		err = fmt.Errorf("expected integer (int64) arguments, got %T and %T values", x, y)
-// 	}
-// 	return
-// }
+func subtract(ctx kern.ExprContext, name string, args map[string]any) (result any, err error) {
+	if len(args) != 2 {
+		err = fmt.Errorf("%s(): requires exactly two arguments", name)
+		return
+	}
+	x, xok := args["a"].(int64)
+	y, yok := args["b"].(int64)
+	if xok && yok {
+		result = x - y
+	} else {
+		err = fmt.Errorf("expected integer (int64) arguments, got %T and %T values", x, y)
+	}
+	return
+}
 
-// func TestEvalStringA(t *testing.T) {
+func TestEvalStringA(t *testing.T) {
 
-// 	source := `a + b * subtract(4,2)`
-// 	args := []Arg{
-// 		{"a", uint8(1)},
-// 		{"b", int8(2)},
-// 		{"subtract", FuncTemplate2(subtract)},
-// 		// force coverage
-// 		{"a16", uint16(1)},
-// 		{"b16", int16(2)},
-// 		{"a32", uint32(1)},
-// 		{"b32", int32(2)},
-// 		{"a64", uint64(1)},
-// 		{"b64", int64(2)},
-// 		{"f32", float32(1.0)},
-// 		{"f64", float64(1.0)},
-// 	}
+	source := `a + b * subtract(4,2)`
+	args := []Arg{
+		{"a", uint8(1)},
+		{"b", int8(2)},
+		{"subtract", kern.FuncTemplate(subtract)},
+		// force coverage
+		{"a16", uint16(1)},
+		{"b16", int16(2)},
+		{"a32", uint32(1)},
+		{"b32", int32(2)},
+		{"a64", uint64(1)},
+		{"b64", int64(2)},
+		{"f32", float32(1.0)},
+		{"f64", float64(1.0)},
+		{"string", "text"},
+		{"bool", true},
+	}
 
-// 	wantResult := int64(5)
-// 	gotResult, gotErr := EvalStringA(source, args...)
-// 	if value, ok := gotResult.(int64); ok && value != wantResult {
-// 		t.Errorf("Source %q got %v, want %v", source, gotResult, wantResult)
-// 		t.Errorf("Error: %v", gotErr)
-// 	}
-// }
+	wantResult := int64(5)
+	gotResult, gotErr := EvalStringA(source, args...)
+	if value, ok := gotResult.(int64); ok && value != wantResult {
+		t.Errorf("Source %q got %v, want %v", source, gotResult, wantResult)
+		t.Errorf("Error: %v", gotErr)
+	}
+}
 
 func TestEvalString(t *testing.T) {