An entity which is denoted by a name we will term <#1436#>elementary<#1436#>; the others are <#1437#>composed<#1437#>. It may very well be that a composed entity at one place of the program obtains a name, by means of a <#1438#>declaration<#1438#>, and at other places of the program is denoted by that name. Such a declaration serves as an abstraction mechanism. Besides a shortening of the program, this naming of entities allows the introduction of various <#1439#>levels of abstraction<#1439#>.