multiline YATL block starting with "=" failing

This YATL block:

```
[[
="a"
="b"
]]
```

evaluates as "a" = "b" and fails on error "SyntaxError: expression cannot contain assignment, perhaps you meant "=="?" 

These alternatives/workarounds work as expected:

```
[[
response.write("a")
response.write("b")
]]

[[="a"]]
[[="b"]]

[[
_=None
="a"
="b"
]]
```