An operator * and a value x form a monoid if * is associative and x is its left and right identity.

In addition suitable contents:
[ = ] [ al ] [ am ] [ an ] [ ar ] [ arc ] [ as ] [ at ] [ ch ] [ ci ] [ de ] [ du ] [ er ] [ era ] [ fi ] [ file ] [ fo ] [ for ] [ gh ] [ h ] [ hr ] [ ht ] [ id ] [ il ] [ is ] [ it ] [ Lex ] [ lu ] [ mo ] [ mod ] [ module ] [ na ] [ no ] [ op ] [ operator ] [ pe ] [ ph ] [ query ] [ rc ] [ re ] [ se ] [ so ] [ to ] [ va ] [ value ] [ ve ]