The + operator and all the comparison operators also work on strings.
The + operator concatenates two string argumennts together, whereas
the comparison operators will compare strings character by character. A
string is less than another if it is earlier in ascii dictionary order and is
greater than another if it is later in ascii dictionary order. The index
operators ' and ` are more complex. The ' takes two
arguments, a string to index (which can be on the stack or taken from a
variable) and a numeric index position. The result pushed is the numeric value
of the character in the string at the index position. The index cannot go beyond
the end of the string. The ` operator is the reverse. It takes three
arguments, the additonal one being the numeric value to write into the string
at the given index position. Below is the definition of substr as an
example of how these operators are used.
%%EQUATE SUBSTR >>E>>S>>S1"">>S2\0>>X\ [<<S<<E<=;<<S'S1\<<X`S2>>S2++S>>S++X>>X]\ 0<<X`S2
The function takes three arguments, the string to produce a substring of, the start index and end index for the string. It pushes the resultant substring.
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