The earliest I've found so far is in "Introduction to PL360 programming" Guertin, Richard L, 1977 at https://www.slac.stanford.edu/spires/explain/manuals/PL360T

The earliest I've found so far is in "Introduction to PL360 programming" Guertin, Richard L, 1977 at https://www.slac.stanford.edu/spires/explain/manuals/PL360TXT.HTML https://www.slac.stanford.edu/spires/explain/manuals/PL360TX... |010| IF R5 = R6 THEN | CR 5,6 | |020| BEGIN | outer block | | BNE T.2 | |030| EXTERNAL DATA BLANKS BASE R9; | L 9,=V(BLANKS) | |040| PROCEDURE SWAP (R7); | B T.1 | |050| BEGIN B2 := B2 XOR B3; | SWAP XC 0(4,2),0(3) | |060| B3 := B3 XOR B2; | XC 0(4,3),0(2) | |070| B2 := B2 XOR B3; | XC 0(4,2),0(3) | |080| END; | of procedure | | BR 7 | |090| B3 := B3 OR B9; | main code | | T.1 OC 0(4,3),0(9) | |100| SWAP; | local procedure call | | BAL 7,SWAP | |110| END; | of outer block | | T.2 EQU * |