String handling functions

These functions are a subset of General functions, grouped here for convenience.

centre() | center()	Return a string centred within its width
getstr()	Return a substring of a longer string
instr()	Search for a pattern in a string
left()	Left-justify a string
right()	Right-justify a string
setstr()	Store a substring within a longer string
strlen()	Return the length of a string
textlength()	Return the number of character positions a string will occupy in a specified font
tolower()	Fold a string to lower case
todate()	Convert a text expression to date
tostr()	Convert an expression to text
toupper()	Fold a string to upper case