anchor.diff:  
     ! Test each string conversion function
     if (all(test_passed)) then
  -     print *,"Test passed." 
  +     write(output_unit,*) "Test passed." 
     else
  -     if ( .not. test_passed(get_double_ )) print *,"In strings.f90, get_double failed."
  -     if ( .not. test_passed(get_real_   )) print *,"In strings.f90, get_real failed."
  -     if ( .not. test_passed(get_integer_)) print *,"In strings.f90, get_integer failed."
  -     if ( .not. test_passed(str_d_      )) print *,"In strings.f90, str_d failed."
  -     if ( .not. test_passed(str_r_      )) print *,"In strings.f90, str_r failed."
  -     if ( .not. test_passed(str_i_      )) print *,"In strings.f90, str_i failed."
  +     if ( .not. test_passed(get_double_ )) write(error_unit,*) "Test get_double failed."
  +     if ( .not. test_passed(get_real_   )) write(error_unit,*) "Test get_real failed."
  +     if ( .not. test_passed(get_integer_)) write(error_unit,*) "Test get_integer failed."
  +     if ( .not. test_passed(str_d_      )) write(error_unit,*) "Test str_d failed."
  +     if ( .not. test_passed(str_r_      )) write(error_unit,*) "Test str_r failed."
  +     if ( .not. test_passed(str_i_      )) write(error_unit,*) "Test str_i failed."
     end if
   
   end program
  
positive.code: program main
    use string, only : get_double, get_real, get_integer, str
    use iso_fortran_env, only : real32, real64, int32
    implicit none
  
    ! Enumerate the test-result array indices:
  
negative.code:   write(integer_string,*) integer_datum
  
    ! Test each string conversion function
    if ( get_double(real64_string)   == real64_datum  ) test_passed(get_double_) = .true.
    if ( get_real(real32_string)     == real32_datum  ) test_passed(get_real_) = .true.
    if ( get_integer(integer_string) == integer_datum ) test_passed(get_integer_) = .true.
  
    ! Left-justify the expected results for comparison
    if ( adjustl(real64_string)      == str(real64_datum)  ) test_passed(str_d_) = .true.
  