constrain behaves weirdly if doubly constraining a parameter:

[richfitz]

Reported by @dwbapst:

library(diversitree)
pars <- c(0.1, 0.2, 0.03, 0.03, 0.01, 0.01)
set.seed(2)
phy <- tree.bisse(pars, max.t=60, x0=0)
lik <- make.bisse(phy, phy$tip.state)
lik.2 <- constrain(lik, lambda0 ~ lambda1, lambda0 ~ mu0)

last line generates warning:

Warning message:
In rels[i] <- res[[2]] : (symbol) object cannot be coerced to type 'list'

but the likelihood function doubly constrains lambda0:

BiSSE likelihood function:
  * Parameter vector takes 5 elements:
     - lambda1, mu0, mu1, q01, q10
  * Function constrained (original took 6 elements):
     - lambda0 ~ lambda1
     - lambda0 ~ mu0
  * Function takes arguments (with defaults)
     - pars: Parameter vector
     - ...: Additional arguments to underlying function
     - pars.only [FALSE]: Return full parameter vector?
  * Phylogeny with 203 tips and 202 nodes
     - Taxa: sp1, sp27, sp29, sp31, sp41, sp43, sp46, sp47, sp48, ...
  * References:
     - Maddison et al. (2007) doi:10.1080/10635150701607033
     - FitzJohn et al. (2009) doi:10.1093/sysbio/syp067
R definition:
function (pars, ..., pars.only = FALSE)  

lik.2(2:6, pars.only=TRUE)

gives

lambda0 lambda1     mu0     mu1     q01     q10 
      3       2       3       4       5       6