Braces { and } indicate repetition (zero or more occurrences), brackets [ and] indicate option<llity (zero or one occurrences), and parentheses ( and) indicate groUT mg (exactly one occurrence) ofthe enclosed expression. 

Within Section 4, EBNF rub describe the formation of lexical symbols from characters; additiona, characters must not occur within a symbol. Sections 5 through 13 use EBNF rules to define the syntax of programs in terms of symbols; s vmbols may be separated by (or preceded by) symbol separators as described in Section 4. 

The term error describes a Prof ram action or state that violates the standard. Any processor may fail 1 () detect errors. 

Implementation-defined mean that a particular Pascal construct may differ between various impll mentations. Each implementation must specify how it implements th It construct. 

Implementation-dependent me,tI1s that a particular construct varies between implementations and that ,Ill implementation does not have to specify how it implements that COl struct. 

An extension is an additional construct not available in all implementations that does not in itself affect or invalidate the constructs of Standard Pascal. Implementations often support extensions in the form of additional predefined and predeclared constants, types, variables, procedures and functions. 

A program that conforms to the standard must not depend on any implementation-dependent constructs or on any extensions. A portable program must, in addition, h' very careful in its use of implementation-defined construct.; (c.g., character set, or range of integer values). 

## 4. Symbols and Symbol Separators 

A program is represented as a seqUt nce of symbols arranged according to the rules of Pascal syntax. Adjal cnt symbols often are separated by symbol separators for purposes of r,.:aclability. Symbols are categorized as the special symbols, identifier" directives, numbers, labels, and character strings. Symbol separat. Irs are spaces, comments, and the ends of lines of the textual prograI I representation. 

