 Alternative notation. It is not necessary to include two kinds of grouping symbols (parentheses and square brackets) in the syntax. Parentheses or square brackets alone would obviously suffice. The two kinds of brackets are employed in this entry for the sake of readability. Given the two kinds of grouping symbols, we could economize further and omit the parentheses from abstraction terms, so that ‘\((\lambda x[M]\))’ would be written as ‘\(\lambda x[M]\)’.