diff --git a/ruleguard/quasigo/compile.go b/ruleguard/quasigo/compile.go
index b9749f5d..c5287eb2 100644
--- a/ruleguard/quasigo/compile.go
+++ b/ruleguard/quasigo/compile.go
@@ -258,38 +258,43 @@ func (cl *compiler) compileIfStmt(stmt *ast.IfStmt) {
 }
 
 func (cl *compiler) compileAssignStmt(assign *ast.AssignStmt) {
-	if len(assign.Lhs) != 1 {
-		panic(cl.errorf(assign, "only single left operand is allowed in assignments"))
-	}
 	if len(assign.Rhs) != 1 {
 		panic(cl.errorf(assign, "only single right operand is allowed in assignments"))
 	}
-	lhs := assign.Lhs[0]
-	rhs := assign.Rhs[0]
-	varname, ok := lhs.(*ast.Ident)
-	if !ok {
-		panic(cl.errorf(lhs, "can assign only to simple variables"))
+	for _, lhs := range assign.Lhs {
+		_, ok := lhs.(*ast.Ident)
+		if !ok {
+			panic(cl.errorf(lhs, "can assign only to simple variables"))
+		}
 	}
 
+	rhs := assign.Rhs[0]
 	cl.compileExpr(rhs)
 
-	typ := cl.ctx.Types.TypeOf(varname)
 	if assign.Tok == token.DEFINE {
-		if _, ok := cl.locals[varname.String()]; ok {
-			panic(cl.errorf(lhs, "%s variable shadowing is not allowed", varname))
+		for i := len(assign.Lhs) - 1; i >= 0; i-- {
+			varname := assign.Lhs[i].(*ast.Ident)
+			typ := cl.ctx.Types.TypeOf(varname)
+			if _, ok := cl.locals[varname.String()]; ok {
+				panic(cl.errorf(varname, "%s variable shadowing is not allowed", varname))
+			}
+			if !cl.isSupportedType(typ) {
+				panic(cl.errorUnsupportedType(varname, typ, varname.String()+" local variable"))
+			}
+			if len(cl.locals) == maxFuncLocals {
+				panic(cl.errorf(varname, "can't define %s: too many locals", varname))
+			}
+			id := len(cl.locals)
+			cl.locals[varname.String()] = id
+			cl.emit8(pickOp(typeIsInt(typ), opSetIntLocal, opSetLocal), id)
 		}
-		if !cl.isSupportedType(typ) {
-			panic(cl.errorUnsupportedType(varname, typ, varname.String()+" local variable"))
-		}
-		if len(cl.locals) == maxFuncLocals {
-			panic(cl.errorf(lhs, "can't define %s: too many locals", varname))
-		}
-		id := len(cl.locals)
-		cl.locals[varname.String()] = id
-		cl.emit8(pickOp(typeIsInt(typ), opSetIntLocal, opSetLocal), id)
 	} else {
-		id := cl.getLocal(varname, varname.String())
-		cl.emit8(pickOp(typeIsInt(typ), opSetIntLocal, opSetLocal), id)
+		for i := len(assign.Lhs) - 1; i >= 0; i-- {
+			varname := assign.Lhs[i].(*ast.Ident)
+			typ := cl.ctx.Types.TypeOf(varname)
+			id := cl.getLocal(varname, varname.String())
+			cl.emit8(pickOp(typeIsInt(typ), opSetIntLocal, opSetLocal), id)
+		}
 	}
 }
 
@@ -557,6 +562,16 @@ func (cl *compiler) compileNativeCall(key funcKey, expr ast.Expr, args []ast.Exp
 	if expr != nil {
 		cl.compileExpr(expr)
 	}
+	if len(args) == 1 {
+		// Check that it's not a f(g()) call, where g() returns
+		// a multi-value result; we can't compile that yet.
+		if call, ok := args[0].(*ast.CallExpr); ok {
+			results := cl.ctx.Types.TypeOf(call.Fun).(*types.Signature).Results()
+			if results != nil && results.Len() > 1 {
+				panic(cl.errorf(args[0], "can't pass tuple as a func argument"))
+			}
+		}
+	}
 	for _, arg := range args {
 		cl.compileExpr(arg)
 	}
diff --git a/ruleguard/quasigo/compile_test.go b/ruleguard/quasigo/compile_test.go
index 8cfe0360..5136b163 100644
--- a/ruleguard/quasigo/compile_test.go
+++ b/ruleguard/quasigo/compile_test.go
@@ -323,16 +323,31 @@ func TestCompile(t *testing.T) {
 			`  PushIntLocal 0 # j`,
 			`  ReturnIntTop`,
 		},
+
+		`v, err := atoi("foo"); println(v); println(err == nil); return err`: {
+			`  PushConst 0 # value="foo"`,
+			`  CallNative 2 # testpkg.atoi`,
+			`  SetLocal 0 # err`,
+			`  SetIntLocal 1 # v`,
+			`  PushIntLocal 1 # v`,
+			`  CallNative 3 # builtin.PrintInt`,
+			`  PushLocal 0 # err`,
+			`  IsNil`,
+			`  CallNative 4 # builtin.Print`,
+			`  PushLocal 0 # err`,
+			`  ReturnTop`,
+		},
 	}
 
 	makePackageSource := func(body string) string {
 		return `
-		  package test
+		  package testpkg
 		  func f(i int, s string, b bool, eface interface{}) interface{} {
 			` + body + `
 		  }
 		  func imul(x, y int) int
 		  func idiv(x, y int) int
+		  func atoi(s string) (int, error)
 		  `
 	}
 
@@ -343,6 +358,15 @@ func TestCompile(t *testing.T) {
 	env.AddNativeFunc(testPackage, "idiv", func(stack *quasigo.ValueStack) {
 		panic("should not be called")
 	})
+	env.AddNativeFunc(testPackage, "atoi", func(stack *quasigo.ValueStack) {
+		panic("should not be called")
+	})
+	env.AddNativeFunc("builtin", "PrintInt", func(stack *quasigo.ValueStack) {
+		panic("should not be called")
+	})
+	env.AddNativeFunc("builtin", "Print", func(stack *quasigo.ValueStack) {
+		panic("should not be called")
+	})
 
 	for testSrc, disasmLines := range tests {
 		src := makePackageSource(testSrc)
diff --git a/ruleguard/quasigo/eval_test.go b/ruleguard/quasigo/eval_test.go
index 8a3da22a..30e25e4f 100644
--- a/ruleguard/quasigo/eval_test.go
+++ b/ruleguard/quasigo/eval_test.go
@@ -14,6 +14,7 @@ import (
 	"github.com/google/go-cmp/cmp"
 	"github.com/quasilyte/go-ruleguard/ruleguard/quasigo"
 	"github.com/quasilyte/go-ruleguard/ruleguard/quasigo/internal/evaltest"
+	"github.com/quasilyte/go-ruleguard/ruleguard/quasigo/stdlib/qstrconv"
 	"github.com/quasilyte/go-ruleguard/ruleguard/quasigo/stdlib/qstrings"
 )
 
@@ -260,15 +261,21 @@ func TestEvalFile(t *testing.T) {
 		}
 
 		var stdout bytes.Buffer
-		env.AddNativeFunc("builtin", "Print", func(stack *quasigo.ValueStack) {
+		env.AddNativeFunc(`builtin`, `Print`, func(stack *quasigo.ValueStack) {
 			arg := stack.Pop()
 			fmt.Fprintln(&stdout, arg)
 		})
-		env.AddNativeFunc("builtin", "PrintInt", func(stack *quasigo.ValueStack) {
+		env.AddNativeFunc(`builtin`, `PrintInt`, func(stack *quasigo.ValueStack) {
 			fmt.Fprintln(&stdout, stack.PopInt())
 		})
 
+		env.AddNativeMethod(`error`, `Error`, func(stack *quasigo.ValueStack) {
+			err := stack.Pop().(error)
+			stack.Push(err.Error())
+		})
+
 		qstrings.ImportAll(env)
+		qstrconv.ImportAll(env)
 
 		var mainFunc *quasigo.Func
 		for _, decl := range parsed.ast.Decls {
@@ -320,5 +327,4 @@ func TestEvalFile(t *testing.T) {
 			runTest(t, mainFile)
 		})
 	}
-
 }
diff --git a/ruleguard/quasigo/stdlib/qstrconv/qstrconv.go b/ruleguard/quasigo/stdlib/qstrconv/qstrconv.go
new file mode 100644
index 00000000..8bc2d943
--- /dev/null
+++ b/ruleguard/quasigo/stdlib/qstrconv/qstrconv.go
@@ -0,0 +1,24 @@
+package qstrconv
+
+import (
+	"strconv"
+
+	"github.com/quasilyte/go-ruleguard/ruleguard/quasigo"
+)
+
+func ImportAll(env *quasigo.Env) {
+	env.AddNativeFunc(`strconv`, `Atoi`, Atoi)
+	env.AddNativeFunc(`strconv`, `Itoa`, Itoa)
+}
+
+func Atoi(stack *quasigo.ValueStack) {
+	s := stack.Pop().(string)
+	v, err := strconv.Atoi(s)
+	stack.PushInt(v)
+	stack.Push(err)
+}
+
+func Itoa(stack *quasigo.ValueStack) {
+	i := stack.PopInt()
+	stack.Push(strconv.Itoa(i))
+}
diff --git a/ruleguard/quasigo/testdata/qstrconv/main.go b/ruleguard/quasigo/testdata/qstrconv/main.go
new file mode 100644
index 00000000..e6901356
--- /dev/null
+++ b/ruleguard/quasigo/testdata/qstrconv/main.go
@@ -0,0 +1,28 @@
+package main
+
+import "strconv"
+
+func main() {
+	s := "16"
+
+	i, err := strconv.Atoi(s)
+	println(i)
+	println(err == nil)
+
+	i2, err2 := strconv.Atoi("bad")
+	println(i2)
+	println(err2.Error())
+
+	println(strconv.Itoa(140))
+	println(strconv.Itoa(i) == s)
+
+	i, err2 = strconv.Atoi("foo")
+	println(i)
+	println(err2.Error())
+
+	i, err2 = strconv.Atoi("-349")
+	println(i)
+	if err2 == nil {
+		println("err2 is nil")
+	}
+}
diff --git a/ruleguard/ruleguard_error_test.go b/ruleguard/ruleguard_error_test.go
index baa6fed0..247cbe74 100644
--- a/ruleguard/ruleguard_error_test.go
+++ b/ruleguard/ruleguard_error_test.go
@@ -82,7 +82,7 @@ func TestParseFilterFuncError(t *testing.T) {
 		// Assignment errors.
 		{
 			`x, y := 1, 2; return x == y`,
-			`only single left operand is allowed in assignments`,
+			`only single right operand is allowed in assignments`,
 		},
 		{
 			`x := 0; { x := 1; return x == 1 }; return x == 0`,