A matrix_column_access_expression retrieves
exactly one column of a
matrix
type, returning a value of an appropriate
vector type representing the column.
For a matrix_column_access_expressione,
the first subexpression is evaluated, by rule
matrix_column_access_pre, and the
resulting expression is a new vector value.
Note that the integer constant given as the index of the column
to return is required by the type rules to be greater than zero
and less than the number of columns in the type, and therefore
evaluation cannot fail due to an out-of-range index.