According to the specification section 4.4.1:
TNS literals MUST be enclosed in square brackets ([ and ]). Each pair of matching brackets MUST introduce a dimension, and nested brackets MUST form a rectangular shape where all sublists at a given depth have the same length. A TNS literal that mixes sub-brackets of differing lengths MUST raise a syntax error.
If an element evaluates to a TNS value, it MUST occupy a single position and MUST NOT contribute additional dimensions to the shape.
In the implementation, however, APPEND constructs the output TNS using the target TNS's static element type, even if the appended value is of a different type. TNS literals, by contrast, compute their element type from their elements (mixed types yield a generic/unknown element type).
According to the specification section 4.4.1:
In the implementation, however,
APPENDconstructs the outputTNSusing the targetTNS's static element type, even if the appended value is of a different type.TNSliterals, by contrast, compute their element type from their elements (mixed types yield a generic/unknown element type).