Looks like fweimer is correct for scheme. I admit to being very surprised. Here is the verbiage from various rXrs:

r3rs, r4rs & r5rs (section 4.2.6):

https://standards.scheme.org/official/r3rs.pdf

https://standards.scheme.org/official/r4rs.pdf

https://conservatory.scheme.org/schemers/Documents/Standards...

"If a comma appears followed immediately by an at- sign (@), then the following expression must evaluate to a list;"

r6rs (section 11.17):

https://standards.scheme.org/official/r6rs.pdf

"If an (unquote-splicing 〈expression〉 . . . ) form appears inside a〈qq template〉, then the〈expression〉s must evaluate to lists;"

r7rs (4.2.8):

https://standards.scheme.org/official/r7rs.pdf

"If a comma appears followed without intervening whitespace by a commercial at-sign (@), then it is an error if the following expression does not evaluate to a list;"

But 2 isn't a list, and the argument to unqoute-splicing must be a list:

> If an (unquote-splicing <expression> ...) form appears inside a <qq template>, then the <expression>s must evaluate to lists; the opening and closing parentheses of the lists are then “stripped away” and the elements of the lists are inserted in place of the unquote-splicing form.