The properties and locations of the data are given in the following structures: ident_data, i_react_data, and r_react_data. The source of the information for these structures is the array nhead/xhead, which is described in Section 3.2.3. Each of the arrays ident_data, i_react_data, and r_react_data is declared in the file `extcor.h', and they are filled by the subroutine rct. Their content is as follows.