The solver will panic if a goal or constraint reduces to a constant expression #77

// lp_modeler 0.5.0
use lp_modeler::dsl::*;
use lp_modeler::solvers::{CbcSolver, SolverTrait};
fn main() {
 let mut problem = LpProblem::new("problem", LpObjective::Maximize);
 problem += LpExpression::literal(0.0);
 let solver = CbcSolver::new();
 let _ = solver.run(&problem); // Here's where the panic happens
}

thread 'main' panicked at 'Requested index out of bound of LpExpression vector. This should not happen.', /home/user/cargo/registry/src/github.com-1ecc6299db9ec823/lp-modeler-0.5.0/src/dsl/variables.rs:368:21
stack backtrace:
 0: std::panicking::begin_panic
 at /home/user/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/panicking.rs:519:12
 1: lp_modeler::dsl::variables::LpExpression::expr_clone_at
 at /home/user/.cargo/registry/src/github.com-1ecc6299db9ec823/lp-modeler-0.5.0/src/dsl/variables.rs:368:21
 2: lp_modeler::dsl::variables::LpExpression::simplify
 at /home/user/.cargo/registry/src/github.com-1ecc6299db9ec823/lp-modeler-0.5.0/src/dsl/variables.rs:567:23
 3: <lp_modeler::dsl::variables::LpExpression as lp_modeler::format::lp_format::LpFileFormat>::to_lp_file_format
 at /home/user/.cargo/registry/src/github.com-1ecc6299db9ec823/lp-modeler-0.5.0/src/format/lp_format.rs:158:25
 4: lp_modeler::format::lp_format::objective_lp_file_block
 at /home/user/.cargo/registry/src/github.com-1ecc6299db9ec823/lp-modeler-0.5.0/src/format/lp_format.rs:61:44
 5: <lp_modeler::dsl::problem::LpProblem as lp_modeler::format::lp_format::LpFileFormat>::to_lp_file_format
 at /home/user/.cargo/registry/src/github.com-1ecc6299db9ec823/lp-modeler-0.5.0/src/format/lp_format.rs:25:27
 6: lp_modeler::format::lp_format::LpFileFormat::write_lp
 at /home/user/.cargo/registry/src/github.com-1ecc6299db9ec823/lp-modeler-0.5.0/src/format/lp_format.rs:12:22
 7: <lp_modeler::solvers::cbc::CbcSolver as lp_modeler::solvers::SolverTrait>::run
 at /home/user/.cargo/registry/src/github.com-1ecc6299db9ec823/lp-modeler-0.5.0/src/solvers/cbc.rs:138:9
 8: tmp::main
 at ./src/main.rs:13:13
 9: core::ops::function::FnOnce::call_once
 at /home/user/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/ops/function.rs:227:5
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

// lp_modeler 0.5.0
use lp_modeler::dsl::*;
use lp_modeler::solvers::{CbcSolver, SolverTrait};
fn main() {
 // adapted from the README example
 let a = &LpInteger::new("a");
 let b = &LpInteger::new("b");
 let c = &LpInteger::new("c");
 let mut problem = LpProblem::new("One Problem", LpObjective::Maximize);
 problem += 10.0 * a + 20.0 * b;
 problem += (500 * a + 1200 * b + 1500 * c).le(10000);
 problem += a.le(b);
 // The problematic constraint
 problem += LpExpression::literal(0.0).equal(0);
 let solver = CbcSolver::new();
 let _ = solver.run(&problem); // panic right here
}