Purpose: Checks whether a value is a string. The value can be of any type but must be declared in case of a variable or object field.

Return Values: Returns true if and only if value is of type string.

Example:

heitml input:	resulting output:
<? isstring(128)>	false
<? isstring(128.39)>	false
<? isstring("256")>	true
<? isstring("256.42")>	true
<? isstring("0")>	true
<? isstring(true)>	false
<? isstring(false)>	false
<? isstring(emptytuple)>	false
<? isstring(null)>	false