fix for in loop symbols

[ozanh]

Closes #288

[d5]

Not part of the change, but let's correct the following comment in the same function:

// ":it" is a local variable but will be conflict with other user variables
// because character ":" is not allowed.

should be

// ":it" is a local variable but it will not conflict with other user variables
// because character ":" is not allowed in the variable names.

[d5]

Great work, @ozanh