can not see the instruction for calling start: in nasm?

Question

I am learning the os-tutorial of https://github.com/cfenollosa/os-tutorial. In the chapter 5. There is one instruction for calling the funciton print: But, after I inspected the inner of boot_sect_print.asm, I didn't see any instructions to call start: which has code for printing the content. Is there any knowledge else for this I missed? Relevant files are as follow:

boot_sect_main.asm

[org 0x7c00] ; tell the assembler that our offset is bootsector code

; The main routine makes sure the parameters are ready and then calls the function
mov bx, HELLO
call print

call print_nl

mov bx, GOODBYE
call print

call print_nl

mov dx, 0x12fe
call print_hex

; that's it! we can hang now
jmp $

; remember to include subroutines below the hang
%include "boot_sect_print.asm"
%include "boot_sect_print_hex.asm"


; data
HELLO:
    db 'Hello, World', 0

GOODBYE:
    db 'Goodbye', 0

; padding and magic number
times 510-($-$$) db 0
dw 0xaa55

boot_sect_print.asm

print:
    pusha

; keep this in mind:
; while (string[i] != 0) { print string[i]; i++ }

; the comparison for string end (null byte)
start:
    mov al, [bx] ; 'bx' is the base address for the string
    cmp al, 0 
    je done

    ; the part where we print with the BIOS help
    mov ah, 0x0e
    int 0x10 ; 'al' already contains the char

    ; increment pointer and do next loop
    add bx, 1
    jmp start

done:
    popa
    ret



print_nl:
    pusha
    
    mov ah, 0x0e
    mov al, 0x0a ; newline char
    int 0x10
    mov al, 0x0d ; carriage return
    int 0x10
    
    popa
    ret