In M, parentheses are used for indexing variables.  Vector elements can be
referred to with 1 index; matrix elements can be refered to with 2 indices. 
Rows of a matrix are selected if only 1 index is used.  A range, e1:e2,
may be used as a row or column index, e.g. A(2:4,:); a plain colon selects all
the rows or columns.  A colon alone as a single index selects all of the
elements unraveled row-by-row to form a single row.