Call a contract from a contract

Dear @fabrice ,
According to the Algorand ABI you can log the result of a contract call.
But then how can you use that result, e.g. when you call a contract from a contract?

There are opcodes that allow reading the logs of inner transaction calls.
(Opcodes - Algorand Developer Portal, with field Logs)

Thank you very much.
I found the following:

txn f
    Opcode: 0x31 {uint8 transaction field index}
    Stack: ... › ..., any
    field F of current transaction

Index	Name	Type	In	Notes
58 		Logs 	[]byte 	v5 	Log messages emitted by an application call (only with itxn in v5). Application mode only
59 		NumLogs uint64 	v5 	Number of Logs (only with itxn in v5). Application mode only
62 		LastLog []byte 	v6 	The last message emitted. Empty bytes if none were emitted. Application mode only

    Opcode: 0xb0
    Stack: ..., A: []byte › ...
    write A to log state of the current application
    Availability: v5
    Mode: Application
	Note: log fails if called more than MaxLogCalls times in a program, or if the sum of logged bytes exceeds 1024 bytes.