.PS
boxht = 0.4

N2: box "F(2)"
box invis "+" wid 0.3
N3: box "F(3)"
box invis "=" wid 0.3
N4: box "F(4)"

N6: box "F(6)" at N2-(0, 1)
box invis "+" wid 0.3
N7: box "F(7)"
box invis "=" wid 0.3
N8: box "F(8)"
arrow from N2.s to N6.n
arrow from N3.s to N6.n
arrow from N4.s to N6.n
arrow from N3.s to N7.n
arrow from N4.s to N7.n

N14: box "F(14)" at N6-(0, 1)
box invis "+" wid 0.3
N15: box "F(15)"
box invis "=" wid 0.3
N16: box "F(16)"
arrow from N6.s to N14.n
arrow from N7.s to N14.n
arrow from N8.s to N14.n
arrow from N7.s to N15.n
arrow from N8.s to N15.n

N30: box "F(30)" at N14-(0, 1)
box invis "+" wid 0.3
N31: box "F(31)"
box invis "=" wid 0.3
N32: box "F(32)"
arrow from N14.s to N30.n
arrow from N15.s to N30.n
arrow from N16.s to N30.n
arrow from N15.s to N31.n
arrow from N16.s to N31.n

N61: box "F(61)" at N30-(0, 1)
box invis "+" wid 0.3
N62: box "F(62)"
box invis "=" wid 0.3
N63: box "F(63)"
arrow from N30.s to N61.n
arrow from N31.s to N61.n
arrow from N30.s to N62.n
arrow from N31.s to N62.n
arrow from N32.s to N62.n

N124: box "F(124)" at N61-(0, 1)
box invis "+" wid 0.3
N125: box "F(125)"
box invis "=" wid 0.3
N126: box "F(126)"
arrow from N61.s to N124.n
arrow from N62.s to N124.n
arrow from N63.s to N124.n
arrow from N62.s to N125.n
arrow from N63.s to N125.n

N249: box "F(249)" at N124-(0, 1)
box invis "+" wid 0.3
N250: box "F(250)"
box invis "=" wid 0.3
N251: box "F(251)"
arrow from N124.s to N249.n
arrow from N125.s to N249.n
arrow from N124.s to N250.n
arrow from N125.s to N250.n
arrow from N126.s to N250.n

N499: box "F(499)" at N249-(0, 1)
box invis "+" wid 0.3
N500: box "F(500)"
box invis "=" wid 0.3
N501: box "F(501)"
arrow from N249.s to N499.n
arrow from N250.s to N499.n
arrow from N249.s to N500.n
arrow from N250.s to N500.n
arrow from N251.s to N500.n
N1000: box "F(1000)" at N500-(0, 1)
arrow from N499.s to N1000.n
arrow from N500.s to N1000.n
arrow from N501.s to N1000.n
.PE