It is our habit in writing an English composition that, as we write each word, each phrase, each sentence, and each paragraph, we consciously or unconsciously check the syntax and the semantics of the composition just written. Writing a computer program in a high-level language could be made similar to writing a composition in English. In this case, a highly interactive highlevel language system checks the syntax and the semantics of the highlevel language program as each symbol, each expression, and each statement are being entered at the terminal. When the source program is completely entered, the program could have been debugged and could have run once.