def StrSubstr(start_idx, count, initial_string):
new_value = initial_string.value[start_idx.value:start_idx.value + count.value]<EOL>return StringV(new_value)<EOL>
Create a concrete version of the substring
:param start_idx : starting index of the substring
:param end_idx : last index of the substring
:param initial_string

:return : a concrete version of the substring