This variant of string is:
string is wideinteger ?-strict? ?-failindex ''varname''? '''stringArgument'''
It returns a Boolean indicator as to whether its string argument is any of the valid forms for a wide integer in Tcl, with optional surrounding whitespace.
In the case of under or overflow in the value, 0 is returned and varname, if provided as an argument to -failindex, will have a value of -1.
[Is there a wiki page that discusses the valid forms for a wide integer? If so, please replace this comment with the page reference. Thank you.]