Variable names starting with underscores

[curio77]

Is there a reason why variable (and presumably function) names cannot start with an underscore (e.g., “_foo”)? Getting an error, “Invalid token: '_'”, when trying to parse an expression containing one into an EvaluableExpression. Underscores are allowed inside variable names, and they are commonly treated like letters in the context of variable names in all programming languages I can think of, so there really shouldn't be a reason.