] gives True if expr is a list, none of whose elements are themselves lists, and gives False otherwise.
] gives True only if test yields True when applied to each of the elements in expr.
NumberQ] tests whether expr is a vector of numbers.
See the Mathematica book: Section 2.3.5, Section 3.7.3.
See also: MatrixQ, TensorRank.
c}] is considered to be a vector as long as none of the entries of the list are lists themselves.
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