diff --git a/v2/precedence_test.go b/v2/precedence_test.go
new file mode 100644
index 0000000..84646e4
--- /dev/null
+++ b/v2/precedence_test.go
@@ -0,0 +1,271 @@
+package arg
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+// this file contains tests related to the precedence rules for:
+//   ProcessCommandLine
+//   ProcessOptions
+//   ProcessPositionals
+//   ProcessEnvironment
+//   ProcessMap
+//   ProcessSingle
+//   ProcessSequence
+//   OverwriteWithCommandLine
+//   OverwriteWithOptions
+//   OverwriteWithPositionals
+//   OverwriteWithEnvironment
+//   OverwriteWithMap
+//
+// The Process* functions should not overwrite fields that have
+// been previously populated, whereas the OverwriteWith* functions
+// should overwrite fields that have been previously populated.
+
+// check that we can accumulate "separate" args across env, cmdline, map, and defaults
+
+// check what happens if we have a required arg with a default value
+
+// add more tests for combinations of separate and cardinality
+
+// check what happens if we call ProcessCommandLine multiple times with different subcommands
+
+func TestProcessOptions(t *testing.T) {
+	var args struct {
+		Arg string
+	}
+
+	p, err := NewParser(&args)
+	require.NoError(t, err)
+
+	_, err = p.ProcessOptions([]string{"program", "--arg=hello"})
+	require.NoError(t, err)
+	assert.Equal(t, "hello", args.Arg)
+}
+
+func TestProcessOptionsDoesNotOverwrite(t *testing.T) {
+	var args struct {
+		Arg string `arg:"env"`
+	}
+
+	p, err := NewParser(&args)
+	require.NoError(t, err)
+
+	err = p.ProcessEnvironment([]string{"ARG=123"})
+	require.NoError(t, err)
+
+	_, err = p.ProcessOptions([]string{"--arg=hello"})
+	require.NoError(t, err)
+
+	assert.EqualValues(t, "123", args.Arg)
+}
+
+func TestOverwriteWithOptions(t *testing.T) {
+	var args struct {
+		Arg string `arg:"env"`
+	}
+
+	p, err := NewParser(&args)
+	require.NoError(t, err)
+
+	err = p.ProcessEnvironment([]string{"ARG=123"})
+	require.NoError(t, err)
+
+	_, err = p.OverwriteWithOptions([]string{"--arg=hello"})
+	require.NoError(t, err)
+
+	assert.EqualValues(t, "hello", args.Arg)
+}
+
+func TestProcessPositionals(t *testing.T) {
+	var args struct {
+		Arg string `arg:"positional"`
+	}
+
+	p, err := NewParser(&args)
+	require.NoError(t, err)
+
+	err = p.ProcessPositionals([]string{"hello"})
+	require.NoError(t, err)
+	assert.Equal(t, "hello", args.Arg)
+}
+
+func TestProcessPositionalsDoesNotOverwrite(t *testing.T) {
+	var args struct {
+		Arg string `arg:"env,positional"`
+	}
+
+	p, err := NewParser(&args)
+	require.NoError(t, err)
+
+	err = p.ProcessEnvironment([]string{"ARG=123"})
+	require.NoError(t, err)
+
+	err = p.ProcessPositionals([]string{"hello"})
+	require.NoError(t, err)
+
+	assert.EqualValues(t, "123", args.Arg)
+}
+
+func TestOverwriteWithPositionals(t *testing.T) {
+	var args struct {
+		Arg string `arg:"env,positional"`
+	}
+
+	p, err := NewParser(&args)
+	require.NoError(t, err)
+
+	err = p.ProcessEnvironment([]string{"ARG=123"})
+	require.NoError(t, err)
+
+	err = p.OverwriteWithPositionals([]string{"hello"})
+	require.NoError(t, err)
+
+	assert.EqualValues(t, "hello", args.Arg)
+}
+
+func TestProcessCommandLine(t *testing.T) {
+	var args struct {
+		Arg string
+	}
+
+	p, err := NewParser(&args)
+	require.NoError(t, err)
+
+	err = p.ProcessCommandLine([]string{"program", "--arg=hello"})
+	require.NoError(t, err)
+	assert.Equal(t, "hello", args.Arg)
+}
+
+func TestProcessCommandLineDoesNotOverwrite(t *testing.T) {
+	var args struct {
+		Arg string `arg:"env"`
+	}
+
+	p, err := NewParser(&args)
+	require.NoError(t, err)
+
+	err = p.ProcessEnvironment([]string{"ARG=123"})
+	require.NoError(t, err)
+
+	err = p.ProcessCommandLine([]string{"program", "--arg=hello"})
+	require.NoError(t, err)
+
+	assert.EqualValues(t, "123", args.Arg)
+}
+
+func TestOverwriteWithCommandLine(t *testing.T) {
+	var args struct {
+		Arg string `arg:"env"`
+	}
+
+	p, err := NewParser(&args)
+	require.NoError(t, err)
+
+	err = p.ProcessEnvironment([]string{"ARG=123"})
+	require.NoError(t, err)
+
+	err = p.OverwriteWithCommandLine([]string{"program", "--arg=hello"})
+	require.NoError(t, err)
+
+	assert.EqualValues(t, "hello", args.Arg)
+}
+
+func TestProcessEnvironment(t *testing.T) {
+	var args struct {
+		Arg string `arg:"env"`
+	}
+
+	p, err := NewParser(&args)
+	require.NoError(t, err)
+
+	err = p.ProcessEnvironment([]string{"ARG=hello"})
+	require.NoError(t, err)
+
+	assert.EqualValues(t, "hello", args.Arg)
+}
+
+func TestProcessEnvironmentDoesNotOverwrite(t *testing.T) {
+	var args struct {
+		Arg string `arg:"env"`
+	}
+
+	p, err := NewParser(&args)
+	require.NoError(t, err)
+
+	_, err = p.ProcessOptions([]string{"--arg=123"})
+	require.NoError(t, err)
+
+	err = p.ProcessEnvironment([]string{"ARG=hello"})
+	require.NoError(t, err)
+
+	assert.EqualValues(t, "123", args.Arg)
+}
+
+func TestOverwriteWithEnvironment(t *testing.T) {
+	var args struct {
+		Arg string `arg:"env"`
+	}
+
+	p, err := NewParser(&args)
+	require.NoError(t, err)
+
+	_, err = p.ProcessOptions([]string{"--arg=123"})
+	require.NoError(t, err)
+
+	err = p.OverwriteWithEnvironment([]string{"ARG=hello"})
+	require.NoError(t, err)
+
+	assert.EqualValues(t, "hello", args.Arg)
+}
+
+func TestProcessDefaults(t *testing.T) {
+	var args struct {
+		Arg string `default:"hello"`
+	}
+
+	p, err := NewParser(&args)
+	require.NoError(t, err)
+
+	err = p.ProcessDefaults()
+	require.NoError(t, err)
+
+	assert.EqualValues(t, "hello", args.Arg)
+}
+
+func TestProcessDefaultsDoesNotOverwrite(t *testing.T) {
+	var args struct {
+		Arg string `default:"hello"`
+	}
+
+	p, err := NewParser(&args)
+	require.NoError(t, err)
+
+	_, err = p.ProcessOptions([]string{"--arg=123"})
+	require.NoError(t, err)
+
+	err = p.ProcessDefaults()
+	require.NoError(t, err)
+
+	assert.EqualValues(t, "123", args.Arg)
+}
+
+func TestOverwriteWithDefaults(t *testing.T) {
+	var args struct {
+		Arg string `default:"hello"`
+	}
+
+	p, err := NewParser(&args)
+	require.NoError(t, err)
+
+	_, err = p.ProcessOptions([]string{"--arg=123"})
+	require.NoError(t, err)
+
+	err = p.OverwriteWithDefaults()
+	require.NoError(t, err)
+
+	assert.EqualValues(t, "hello", args.Arg)
+}