Operators '??' and '?=' added

parser_test.go

@@ -132,6 +132,11 @@ func TestParser(t *testing.T) {
 		/* 111 */ {`double=func(x){2*x}; a=5; two=func() {2}; (double(3+a) + 1) * two()`, int64(34), nil},
 		/* 112 */ {`import("./test-funcs.expr"); (double(3+a) + 1) * two()`, int64(34), nil},
 		/* 113 */ {`import("test-funcs.expr"); (double(3+a) + 1) * two()`, int64(34), nil},
+		/* 114 */ {`x ?? "default"`, "default", nil},
+		/* 115 */ {`x="hello"; x ?? "default"`, "hello", nil},
+		/* 116 */ {`x ?? func(){}"`, nil, errors.New(`[1:15] the right operand of a coalescing operation cannot be a function definition`)},
+		/* 117 */ {`x ?= "default"; x`, "default", nil},
+		/* 118 */ {`x="hello"; x ?= "default"; x`, "hello", nil},
 	}
 	check_env_expr_path := 113