`format_parameters()` edge cases

library(plm)
data("EmplUK", package = "plm")

## Arellano and Bond (1991), table 4 col. b 
z1 <- pgmm(log(emp) ~ lag(log(emp), 1:2) + lag(log(wage), 0:1)
           + log(capital) + lag(log(output), 0:1) | lag(log(emp), 2:99),
           data = EmplUK, effect = "twoways", model = "twosteps")

parameters::format_parameters(z1)
#>             lag(log(emp), 1:2)1             lag(log(emp), 1:2)2 
#>    "lag(emp [lag(log, 1] * 2)1"    "lag(emp [lag(log, 1] * 2)2" 
#>            lag(log(wage), 0:1)0            lag(log(wage), 0:1)1 
#>   "lag(wage [lag(log, 0] * 1)0"   "lag(wage [lag(log, 0] * 1)1" 
#>                    log(capital)          lag(log(output), 0:1)0 
#>                 "capital [log]" "lag(output [lag(log, 0] * 1)0" 
#>          lag(log(output), 0:1)1                            1979 
#> "lag(output [lag(log, 0] * 1)1"                          "1979" 
#>                            1980                            1981 
#>                          "1980"                          "1981" 
#>                            1982                            1983 
#>                          "1982"                          "1983" 
#>                            1984 
#>                          "1984"

^{Created on 2021-05-26 by the reprex package (v2.0.0)}

Start by reproducing the parameters::format_parameters(z1) example from the issue with the plm package and EmplUK data. Inspect how the displayed lag() and log() expressions are formatted, then use the issue's expected edge-case behavior to define corrected parameter labels and verify the output no longer contains malformed expressions.