10. In a computer system comprising a computer, a method of annotating computer program code stored on a computer-readable medium, wherein the computer program code is operable to cause computer execution to perform according to instructions in the computer program code, the method comprising: inserting, by the computer, an annotation at a first value having a first value type in the computer program code; wherein the annotation comprises a first instance of a keyword, the first instance of the keyword indicating that the first value satisfies all usability properties necessary to allow a first function to rely on the first value, wherein other instances of the keyword identical to the first instance are operable to indicate that other values having different respective value types satisfy all usability properties necessary to allow functions to rely on the respective other values, wherein use of the keyword associates a pre-determined set of usability properties with a value type, and wherein the usability properties depend on the value type.
10. In a computer system comprising a computer, a method of annotating computer program code stored on a computer-readable medium, wherein the computer program code is operable to cause computer execution to perform according to instructions in the computer program code, the method comprising: inserting, by the computer, an annotation at a first value having a first value type in the computer program code; wherein the annotation comprises a first instance of a keyword, the first instance of the keyword indicating that the first value satisfies all usability properties necessary to allow a first function to rely on the first value, wherein other instances of the keyword identical to the first instance are operable to indicate that other values having different respective value types satisfy all usability properties necessary to allow functions to rely on the respective other values, wherein use of the keyword associates a pre-determined set of usability properties with a value type, and wherein the usability properties depend on the value type. 14. The method of claim 10 wherein the annotation further comprises an except qualifier.