i have a conditional expression like this 

for i from ... do

if .... then
exp1:
exp2:
exp3:
....... 

else 

exp4:
exp5:
exp6:
.... 

end if;

od;

i face this error during caculation:
Error, reserved word `else` unexpected

what should i do?

> with(PDEtools):
> declare(y(x), prime=x):
> ode := diff(diff(y(x),x),x)*diff(y(x),x)*y(x)*f(x)-2*diff(y(x),x)^3*x^6> + 2*diff(y(x),x)^2*y(x)*diff(g(x),x) + y(x)^5; 
 diff(ode,diff(y(x),x));
Error, invalid input: diff received diff(y(x),x), which is not valid for its 2nd argument

 i want to make deravatition with respect to the first deravatives,what should i do? 

i have three nonlinear pde equations and i want to solve them,with any initial or boundary condition possible,can anyone help me solve them?
it is of no importnace to solve them numerially or not. 

i had 3 nonlinear equations and i want to convert them to the form of multiplications of a MATRIX in a VECTOR, but none of these two methods give me the right answer,what should i do?

#method one#

restart:
with(LinearAlgebra):
eq1:=a[1]+a[2]*a[3]^2*a[1]+a[2]*a[3]-a[1]^2:
eq2:=a[3]+a[1]^3+a[2]*a[3]+a[2]^2-a[1]^2:
eq3:=a[1]^3+a[2]^3+a[1]*a[2]*a[3]+a[1]*a[2]-a[3]:
for i to 3 do
B || i := eq || i ; K || i := Vector[row](3):
for j to 3 do

how can i define a matrix as a sunction of a variable so that when i chande the independet variable,the elements of the matrix also change?

A(t):=[a[t],b[t],c[t]] ; i want to define a function like this,so that by changing the time t,the elements of the matrix also change,what should i do?